Old crush reaching out even though he has a girlfriend

So I’m a little confused and I kinda wanted some insight from you guys.

I was in this situation YEARS back, like I’m talking close to 6 years where me and this guy both had a crush on each other but we were both way too shy to make a move. (I found out about this a little later because I knew I had a crush on him and thought he was way out of my league only to find out he was crushing on me back but was also too shy)

Anyway, we were both basically low tier friends in this mutual group, meaning we weren’t in the core middle but we met through them. Basically I had a falling out with them due to forcing to pick sides bleh bleh blah stupid drama.

Because of that fall out, I expected that I was never going to see him, let alone hear from ever again and I accepted that completely. Flash forward, he has been dating this girl for a while now but added me on social media via number so he still has my number which I was surprised (when someone adds you, some apps tells you how they found you)

And now he randomly reached out and wanted to catch up and it was nice and all but I wasn’t sure if he became single? I played it as if I thought he was still in a relationship and as the conversation stretched back and forth to a couple of days I basically told him “hey so I want to respect you and your relationship, I hope this is just as friends. I don’t want to cause any trouble.”

He messaged me saying that it is purely platonic and it is just as friends, thanking me for respecting the boundaries. But then continues with lengthy conversations everyday this far. Am I bugging or is this strange...?

Someone mentioned it is because of the Quarantine Boredom. And I speculated that as well.. but we were never THAT close to the point of talking via messages even back then, even though we both had a crush on each other.