Hi! Netmaker here. At the time of this writing this is true, but we're making some licensing changes this sprint, which I think will make people very happy. We started with SSPL just because it's much easier to go from more restrictive to less restrictive, as opposed to the alternative.
However, several months ago we moved all of the client-side code to Apache-2.0, and are about to make the server-side code FOSS-compatible.