Comment by OJFord

Oohh, this is exactly why you can't. I just commented similarly, but yours made me realise - this must be an agreement between the companies, Mullvad doesn't want you to fairly easily have all devices on the same tailnet, single exit node using 1/5 keys on Mullvad. Without Tailscale, if you configured them all separately, it'd use as many keys as you had devices.

You can similarly bypass it without Tailscale, the same way you had to do it in Tailscale before this announcement, with everything egressing via a server which is the single Mullvad client. But it makes sense with the built-in solution (with probably better latency etc.) that they wouldn't want that.