In my experience you can do reasonable bots cheaper than sending network updates to a regular player. Thats just straight up, but you can also tick their logic way less than every update if you want. Even more way-less if nobody is near them or spectating them.

Also some stuff you might want to calculate for them to use to make decisions can be shared among all bots.

As I aluded to in my post, how expensive the AI is depends entirely on how complex and optimized is. Remember that the process of encoding and sending packets to a client and receiving/decoding/processing the client's packets in turn is completely skipped for 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.

An advantage to bots is that the server can trust them. It doesn't need to do any verification of input since it can trust that they aren't cheating (except they probably are...).