I wonder how people come up with the bot superstition? Just a feeling or is there any valid indication of massive influx of bot accounts?

Experience, mainly.

I used to run a phpbb forum, on average the bot signups outnumbered the real people 10 or 20 times. And that was with some fairly robust anti spam measures in place - something I think this platform is too new to have properly sorted out yet.

I may be wrong, I don't know how the back end here works, but any place where people can post publicly will be infested with bot signups very quickly. The only real variable is how good the anti spam measures are.

What is something someone can gain by swarming an instance or forums like yours with bots? I cant wrap my head around it. Also if someone has an instance and swarms it with bot accounts, it may seem like you got a popular instance but where is the revenue if there are noone who is able to click an ad? Do they do it just for the lols?

Spin up 50 bots.
Sign them all up for lemmy.
Let accounts interact/age.
Sell accounts to companies who want to advertise as one of the cool kids.

Happened on reddit nonstop.