The way it works is this: You can sign up for a proxy rotator service that works like a regular proxy except every request you make goes through a different ip address. Is that a botnet? Yes. Is it also typically used in a scraping project? Yes.
It can absolutely be that, but that requires a confluence of multiple factors - misconfigured scrapper hitting the site over and over, a big bot net like proxy setup that is way overkilled for scrapping, a setup sophisticated enough to do all that yet simultaneously stupid enough to not cope with a site is mostly text and a couple gigs at most and all that over extended timeframe without anyone realising their scrapper is stuck.
Except that I think it's clear that the motive was getting the data not taking the site offline. The evidence for that is that it stopped on its own without them doing anything to mitigate it.
Also I don't know why you think this is sophisticated, it's probably 40 lines of Python code max.
