I must say, I think people definitely overrate this Raptors team. They have great depth, but no true number 1 option. Siakam is really good, but he just isn't great enough to be the lead scorer on a championship team. Not this year, at least. After that really hot start Siakam hasn't been as good as his numbers indicate.
I don't believe most people overrate the Raptors. If anything, they were heavily slept on to start the season and people gradually began to respect their display of continuity in the face of losing Kawhi. Maybe I'm wrong but I'd be surprised if most people view the Raptors as true title contenders. As you stated, they lack a clear cut top tier player.
That being said, they are going to be a tough out for any team in the Eastern Conference. The Celtics were awesome last night but Toronto didn't show up with anything close to playoff intensity. At the end of the day the Celtics don't have a LeBron, Kawhi, Giannis etc., which means they're still vulnerable against a team like Toronto.
I'm still taking the Celtics in a 7 game series but it could definitely go either way. I also believe the Celtics have a shot to win the title. Our margin for error is just smaller than the Bucks, Lakers, and Clippers.