It doesn't look like Redis requires a copyright assignment from its contributors. The bulk of the files have a copyright statement for Salvatore Sanfilippo (antirez), but other contributors have copyrights in there as well (including Redis Labs). Redis Labs employs antirez and bought the trademark from him, but it doesn't appear that he transferred copyright to them. Even if he had, I don't think Redis Labs would be able to unilaterally relicense Redis.

What Redis Labs owns (and is distributing under this new proprietary license) is a set of modules that they developed for use with Redis. They aren't officially part of Redis.

Yeah, it's a very ugly and confusing case of "trademark overload" where "Redis™ Labs" is not directly the owner of "redis" the server application but is directly the owner of "Redis™ Modules" add-ons for the server application. Redis™ is not exactly "redis", one of these things is not like the other.