Whatever the forum software Woltlab Burning Board considers a user. If I recall correctly, it tries to build an identifier based on PHP session ids, so most likely simply cookies.
Couple that with 15 minute session times, and that could just be one entity scraping the forum at 30 requests per second. One scraper going moderately fast sounds far less bad than 29000 bots.
It still sounds excessive for a niche site, but I'd guess this is sporadic, or that the forum software has a page structure that traps scrapers accidentally, quite easy to do.