Maybe a dumb question but since 2B2T (as I understand it) is a single instance of everyone modifying it to their heart's content, why does the download look like the standard biomes of a fresh minecraft map?

It is running on a single machine so the player count is relatively low. Today the limit is ~1500 concurrent players but it used to be much lower. And of the players online probably half are bots which don't build.

The map is 30Mx30M blocks so outside of the very center you'll not see that much player activity.

DonutSMP is the largest Minecraft server today, the overworld is 225,000x225,000 blocks and the map is modified wherever you look.

The world is mind bogglingly big; as are most minecraft worlds, but this one especially.

People sprawl far to prevent PvP encounters.

Besides the spawn area, most blocks have never been touched. It's a lot of blocks.

You gotta zoom into the center to see the main chaos. And if you zoom in elsewhere, you can see all the generation differences from different versions of the game.