Subject: It shouldn't
Posted on: 2015-09-18 15:37:20 UTC
A new post is a reply created less than 24 hours ago to a post created more than 24 hours ago.
Actually, these guys should eventually show up as "New", but we'll have to wait a bit.
EDIT: I thought I'd mention that "watch thread" does in fact watch the entire thread, not just immediate replies. That's why it's off by default.