Heh, I understand the sentiment but he does have a point.
You still do have to actually MAKE this monstrosity. If you have three hundred encounters in your mega-dungeon, you're looking at an insane time sink to actually prep that. Keeping your mega-dungeon under a reasonable limit is good advice. That level 1 Undermountain map is insane. It would take me freaking years to actually detail that out for play.
Heck, my World's Largest Dungeon adventure is like 1000 pages long. It's ludicrous. Glorious but ludicrous.
Keeping things to bite sized chunks so that you can actually make the thing is a good idea.