This is unfortunately true, at least to some extent.

However, I still believe that partly open sourcing, or dual/triple licensing Sailfish OS could be a wise move. Nemo and Mer are a bit far from functional product people use every day so it's no wonder there is limited interest in them even if effort put into them also benefits Sailfish. Mere licensing change won't cut it though, there would need to be an infrastructure and processes to support co-development, accepting patches, documentation etc. And of course there is the hurdle with some parts being kept out of public (in regard to future products, etc). But still, IMO it would be the best way, now that the foundation and UI is more or less solid.