I've reached out to OutSystems, and I hope they will reply. I also talked to someone from an official OutSystems training partner, and they said "Official training materials not available on the OutSystems website are only licensed to Training Partners and there are royalties and the likes involved" - so training partners have official training material.
It seems to me that it's a bit shady to ask money for training that involves the free OutSystems material, but since we do not know how OS feels about this, I'll let them comment on this.