Are tildes federated? Can I engage with tildes from my mastodon account as I do currently with lemmy's or kbin forums?

Tildes is centralized. There's no federation.

I would say federation is a must as it will spare us from the next migration (there is a rule: centralized or proprietary services *always* get worse as time goes by).

I think Tildes is pretty good, but their subreddit-equivalent policy is bizarre—only admins can make them, and they will only make them if they see demand. This is a chicken-and-egg problem that will prevent many niche topics from finding a community, the way they did on Reddit. I hope it's just marketing talk covering a technical limitation instead of a genuinely permanent flaw.

What I know Tildes does not want to be a full Reddit replacement. I do find their model of moderation better than Reddit. I have already seen some Reddit mods asking about moderating communities in Tildes.