Not sure either tbh. It says “If you are not connected to Mullvad VPN then the Mullvad extension will recommend that you enable Mullvad DoH.”, and then proceeds to say “It is better to use our non-logging privacy DNS than your Internet provider’s DNS.”, so it’s clearly not about other VPN users.
My guess is that it’s probably better anti fingerprinting wise to keep it on, but keep in mind that you’re trusting both Mullvad and Windscribe