I agree with @NeuroticFish, spam isn't only define because of how many pages or posts has been posted on the thread, but it's mostly because of the post quality. The judgement to know how high the quality is from both campaign managers (if the user join a signature campaign) and the moderators (which they has a power to delete someone post). As long as the campaign managers count his post and the moderators didn't delete his post, this mean his post has decent quality and not a spammer.
Just because the manager counts a post and the moderator doesn't delete it doesn't mean the post is actually good. Most of the time, our reports go unhandled because there are so many of them. Additionally, some managers use Ninjasty.space to count posts without properly evaluating the posts' quality as long as the user met the weekly post requirement.
Why do you specifically refer "spam" to a mega thread?
No, but home to spammers
Do you think every person who posts on the mega threads is a spammer?
Not all, but most are.
As far as I understand, spam is everywhere, it is not specifically located in mega threads only.
Please don't mislead people to judge "spam" is always related to mega threads.
I'm not misleading anyone but most mega thread contains a lot of spam, compared to smaller ones.