Kitchen cabinets dont usually have to hold up 1500+ pounds. with his 100 gal, if he uses even a 15 gallon sump, that's 1000 pounds of just water. add aquarium weight, sand, rock, and other equipment...
While plywood is acceptable for holding up the weight of a reef aquarium, 2X4s are fine, and you won't have to worry about how strong it is. I know that my 90 gallon stand will support 2500 pounds, where as the plywood stand that I bought for my 55 gallon buckled (but that's OK--it's how I got my 90). That is why I don't trust plywood.
A lot of people here on RC are pretty much beginners (me included) at woodworking, so one unrealized mistake in a plywood stand and several thousand dollars are sitting in a pile of broken glass and water on your floor ruining several thousand dollars worth of carpeting. that is the other reason for "DIY overkill"