You just put a txt record in your DNS to get a Bluesky username. They don’t have to manage your domain

Edited for clarity

Presumably you'd transfer your domain from Namecheap, not bluesky; there are extremely strict conditions on registrars around domain portability.

This is actually really messy.

It’s unclear to me who is the registrant in this scenario.

If blue sky then you don’t own anything and there is no portability.

If you are the registrant that’s great but namecheap is going to need contact information that’s verifiable which may turn people off who would like a bit more separation on their social profiles. I also wonder does blue sky see that registrant data? Can’t say I like that very much.

It works in a similar way that lets encrypt DNS challenge works. They just partnered with Namecheap to make the process simple because asking people to buy a domain and setup TXT records is still a fairly complicated and technical process. Apple does the same with iCloud emails and Cloudflare domains.