Except that IS the spam strategy.
As TechnoBear can definitely attest to, even with strong anti-spam bot code, every forum has to deal with spam that gets through the filters. Consider a filter, an email check, etc, as being a % of spam blocked. That’s great, but when you start looking at moderation requirements, it’s not a percentage game - every individual piece of spam has to be handled by a moderator, which takes X amount of time, which means its a numbers game - and the bigger or more popular the forum, the bigger those numbers are.
PhpBB is not good at anti-spam (part of the reason, IIRC, that this forum moved AWAY from such technology…) and FileZilla is a big enough project that their forum will be a fairly high value target for spammers, as well as volume of legitimate users. So either you have a lot of moderators, whose sole job is to hunt the hundreds, if not thousands, of spam messages that % represents… or you have to get stricter in enforcement.