That article you linked doesn't mention Xochimilco at all so I have no clue what you're quoting. I can't find a single source for your 80 tons claim (other than some blog post that cites another blog post), which if true and I suspect it isn't is 20 tons less per hectare than many conventional vegetables like cabbage and tomatoes. Other sources I found cite a number that's less than half of what you're claiming. Do you have a real source that isn't a blog post?

>Post-industrial agriculture is not actually more productive per area. It's just more productive per input labor.

This is alarmingly false. As I pointed out many conventional vegetables yield 100 tons per hectare today. Moreover yes they are more productive per unit of labor. The Mexica and their contemporary polities around Lake Texcoco were miserable slave societies that used armies of captured war salves (tlacotin) to perform much of the work. They also used unpaid corvee labor through the coatequitl system, and serfs known as mayeques. So honestly its quite the social advancement that we don't have to press people into agricultural labor at spear point anymore.

> Agricultural yields within the most densely populated and productive preindustrial land-use systems compared well with modern yields

The references for this quote are about South East Asian rice agriculture, which today is still done more or less done the way it was in premodern times. This quote doesn't support your argument and is at best deceptive.