I see, I misunderstood that. I have read it as an attempt to prevent redis taking the code from valkey.
However, if the intention was the other way (to allow valkey to take code from redis), valkey should just go for AGPL as well, there is little reason to pick GPL if the code sharing would be the motivation for the license change.
Disclaimer: I am not a lawyer, this is not legal advice.
In theory, one would not be able to offer a combined program under other licenses (in particular, RSALv2 and SSPLv1), as those licenses have conflicts with GPL obligations.
Direct contributions to the Redis project avoid this issue via a separate Contributor License Agreement. It would only mean that Redis developers could not unilaterally copy code from Valkey.
I'm not saying that the Valkey community should do this. Personally, I think it's better off as a BSD-3 licensed project, with the community fulfilling the promise made by others that it would always be that way.
Some code under a 3 clause BSD and some under AGPLv3 could be interesting.