Well no, but his suggestion (perhaps more implied than stated) that d20 makes "winging it" easier DOES argue for the merits of the system. I like a system that, if I make up a rule on the spot using my knowledge of the basic concepts of the game, I end up with a mechanic that is more or less identical to the one I couldn't remember.
That's a good point, and one I concede given D20's one resolution mechanic, but in my opinion that's aiming a bit low, on two counts:
1) This appeals to the now-traditional sophistry where somebody says "D20 ain't nothin' but dice n' adds" to respond to an aspect of the system not working well for somebody. Obviously, the issues come up a few steps of resolution higher than that, and pretending they don't doesn't wash.
This matters because there are spots where winging it wrong will seriously change what a character might accomplish. Missing an AoO is a big deal for characters with Combat Reflexes; missing a flank is a big deal for a character with Sneak Attack. Missing certain monster capabilities will sometimes completely alter their effective CR.
2) One resolution mechanic in games has been a principle that's been around since the 80s and before. Saying you can wing a system with one resolution mechanic more easily is not really saying much in defense of the system at all, except that it was designed in the last 25 years.
Inventing "Bull Rush" on the fly is going to get you something VERY similar to the existing rule, if you're familiar with attacks of opportunity, opposed checks and so on. Likewise just about any special combat action except for turning undead, which drives me crazy.
That's because turning undead pretty much sucks

As for something like Bull Rush, the problem is presentation. Bull Rush and Trip both demonstrate a design principle in the game that could just as easily be stated outright instead of written as discrete systems.
Sure you can wing any game, if you're not concerned with whether or not it will be rewarding and fun to play. One of the things I like about d20 is that when I wing it, I end up with pretty much the same game as when I know the rules.
I've run fun games with terrible systems. That doesn't make the system sudddenly not-terrible. It just meant that fate and circumstances led to me using those rules and winging away what I didn't care for. If I drop turning because it sucks and wing a better system, this does not argue for the merits of the system.
What good I think can come of these discussions is to come up with workable variations instead of just telling people they're on their own. The iteration of this argument I despise the most is when people want to play certain characters, but the rules for those characters suck, and are told that "It's not the game's fault you wanna play something that sucks." That humdinger gets used in 90% of discussions on multiclassing.
Incidentally, I don't think 3e sucks, myself. What I do think is that it's designed to promote a certain kind of approach and, being the top game, gets used for others -- and sometimes disappoints when taken beyond its designed range.
Whether or not you think that's rewarding and fun is of course, up to you, but at least it's consistent. Somebody who LOVES Phoenix Command is probably going to have less fun if your winging it has little relationship to the rules. Other people might have MORE fun, of course, but for this guy, all the effort he's put into learning the rules is going to waste every time you make a ruling that is in opposition to the actual rules. Which might make it less fun for him -- and maybe even for you.
This is rather my point. Rules-adherent play in D20 is *supposed* to encourage people to invest time mastering the rules and molding themselves to the game (and I didn't just say this; Monte Cook did). Not everyone enjoys things this way. I enjoy things this way half the time; the other half I play a different RPG.
So there is an argument to be made that systems that make it easier to wing it and still approximate the correct rules are in fact more likely to be fun to play.
I don't disagree -- but again, the problems people run into are not with fundamentals, but with details. Sometimes those details are very important to the overall cohesiveness of the system.
For example, Concentration checks are, in my experience, far and away one of the most often forgotten and annoying rolls to make. They exist solely as a balance artifact for team play, and solely as a punitve measure for spellcasters. It disinclines players to remember than and it makes DMing annoying when you have to tell people to roll to see if they get hosed. Drop them, though, and spellcasters really get a boost above and beyond what they ought to have.
Now, some people will love the idea of mastering a system well enough and tightening group play to the point where these considerations aren't an issue at all. That's fine. But the folks who go "ah, screw this" early in the game and want a way to fix that are *also* right.