I think it has absolutely nothing to do with Reddit and everything to do with Twitter.
I think they scrambled to get something up and running quickly so they could get the wave of disgruntled Twitter users and jumpstart a new social media for them, and the only feasible option in 5 months was to use Mastodon/Activitypub to get there.
It will be interesting to see how much they give back to the community and if they federate.

They have 30 something mods, they are 4 admins.
It’s not about being selfish, it’s about creating a space that they think can be a safe space. That’s what’s cool about federation, you just use other instances and that’s it.

I’m sure it’s coming soon enough. The federation is such a great concept, interoperability is great!