Not true. If you enable mullvad in tailscale then when you have tailscale installed and enabled on your phone it allows you to route your phone through mullvad which has been enabled in the tailscale console. Its true about only one vpn connection, but with tailscale it provides mullvad connections in the app if you enable it in the tailscale console. Then you can access your containers remotely through tailscale and also have all your internet traffic go through mullvad all in the tailscale app