Nowadays it's more like 20 players and 80 bots, so a lot less networking stuff going on, and the bot AI is so basic that I doubt it has a significant impact on server performance unless it's very badly implemented.
But it wouldn’t surprise me too much if those use a higher tier of hardware.
Also some stuff you might want to calculate for them to use to make decisions can be shared among all bots.
Fortnite bots are very barebones and are only capable of performing a handful of simple tasks in repetitive ways. It's entirely plausible that the code responsible for governing their actions is fast enough to be less expensive than networking a real player.