My problem with this guide is that it's a one-sided presentation that repeatedly discusses the evils of GM prep and the joys of zero prep while glossing over the potential pitfalls of zero prep and how to avoid them. Besides, it's still asking you to do some prep.
Consider this core statement: "Random tables eliminate the need for session prep." They don't. For one thing, you need to come up with the tables in the first place. That's prep time even if you go looking for published tables, and if you spend any time reading them or thinking about them before you play. Also, do you know the Birthday Problem? It calculates the probability that no two people in a group have the same birthday. Do the math and it turns out that in a group of 23 or more people, two people sharing a birthday is more likely than no two people sharing a birthday. And birthdays are essentially a d365 table.
In a d100 table, which they seem to favor over at dicegeeks, that crossover point happens at the 12th roll. That is, by the time you've rolled 12 times against a d100 table, repetition of a previous roll is more likely than not. Ask yourself how often you'd roll on a given table. Multiple times per room that the PCs enter? Once per encounter? Once every 15 minutes of session time? Then figure out how long it'll take you to reach that 12th roll. That's when repetition gets likely. "You find another telescope" (or whatever you're rolling up randomly) gets less interesting with every repetition. Your fifth telescope doesn't mean you're having five times as much fun. If you can live with the repettion your d100 table would give you, great. If not, you've got a problem.
What's the fix to avoid repetition? "Roll again" isn't a good approach because you'll do it more and more as you use up a table. This wastes session time and can kill the flow of play. "Uh, hang on, we've done telescopes already. [Roll] And rusty swords. [Roll] Have we done sundials?" The fix is not to use the same table(s) over and over and over. Instead, use tables that reflect the different locales and environments in your setting. How do you do that? Prep time. Maybe you can find a variety of tables in books and online sources, or maybe you'll make up your own, but that's still prep time. It's not wasted time if it helps you and your players have fun, but it's still prep time.
The guide claims that if you prepare something the players never encounter, you've wasted your time. That doesn't have to be the case. Sure, it's a waste to roll up detailed room contents for a zillion rooms when the PCs will hit only an unpredictable fraction of them. I'd consider it a waste even if they visited every room, because "there are cobwebs, a table, and three wooden chairs" gets old pretty fast. Instead, focus on your process instead of making an unthinking series of dice rolls. If you have 10 minutes to prepare, roll up three things, and ponder how they might be related. Suppose you get a rusty sword, a goblin, and a rickety bridge across a chasm. What's special about this rusty sword? Why is it here instead of elsewhere? Is it lying around loose or is it hidden away? What's the goblin's interest in it? Why is the bridge here? What's on either side? Why is it rickety? What does the bridge have to do with the sword? You don't have to force your answers on the PCs, who might come up with their own ideas you can run with, but you've still done a useful warm-up exercise. You've primed yourself for improvising during the session.
The "zero" prep method in this guide isn't zero prep. It tells you to gather ideas, watch movies and TV shows, read books, listen to audiobooks, get familiar with story structure, search online for maps and pictures and whatnot, find a selection of random tables, and set up a laptop for use during play. That's all prep time, not zero prep.
"Use Combat to Stall" is potentially a bad idea. A combat should be exciting and interesting and relevant, not a time burner to cover up for a lack of preparation. Start the session 10 minutes later if you need a little prep time, instead of wasting an hour on a combat that serves no other purpose. Besides, if you're busy managing a combat, it'll be harder to come up with ideas. What happens when the players catch you off guard during play? Instead of deliberately stalling, use something from those extra 10 minutes you took, or make the players part of the solution instead of treating them like someone to be distracted while you come up with the answer. You can say, "You got me, so let's make something up."
There are three reasons to avoid or minimize prep time: 1) You just plain don't have the time. 2) You don't enjoy it. 3) It's not helping you during play. Instead of trying to eliminate prep time entirely, try to focus on the fun parts of session prep, and use it as preparation to improvise instead of just cranking out unnecessary detail. Focus on a few critical things that can help you during play (quality over quantity). A little time spent on good prep is much better than wasting session time with rerolling until you're happy or deliberate stalling. If you really want to avoid prep time altogether, use a GMless system, or let someone else be the GM. Otherwise, even a heavily improvised session involves preparing to improvise.