From the article:

Since Tailscale was founded in 2019, customers have been forced to choose between either Tailscale or Mullvad without the ability for them to co-exist.
Today we announce a partnership with Tailscale that allows you to use both in conjunction through the Tailscale app. This functionality is not available through the Mullvad VPN app. This partnership allows customers of Tailscale to make use of our WireGuard VPN servers as “exit nodes”. This means that whilst connected to Tailscale, you can access your devices across Tailscale’s mesh network, whilst still connecting outbound through Mullvad VPN WireGuard servers in any location.

Announcement also on Tailscale blog.