I played on several teams that had several matches where the other team could have ffw'd us. Every single time it happened, every opponent I've ever had has tried as hard as they could to avoid the ffw. I've never seen a team that wasn't willing to wait as long as they possibly could. It's certainly understandable to say "look, it's 25 minutes past the start time, you don't know when your teammates are coming, and we have to go soon, so we'll give you another 2 minutes and then we're calling it." That's entirely fair. But if it's exactly 15 minutes past and the other team is in the process of joining... claiming the ffw in that situation is incredibly rare in this community. Unheard of, even.
I mean, this is a community where if a player drops, the other team will refuse to push until they reconnect.