That was exactly my reaction. Seems like some kind of publicity stunt by both companies.
https://github.com/tailscale/tailscale/issues/2452 links to all the client commits.
There was also a lot of control plane stuff to let you specify config for different NextDNS profiles per tags/groups/users/etc.
And NextDNS improved some things on their side too, especially around passing device info to them.
And then, yes, we publicized it after working on it. And closed that bug.
The built-in resolver at 100.100.100.100 handles the *.your-tailscale-suffix names in your tailnet and the rest that doesn't match goes upstream. Basically.