So?
I don't think that, from a technology-transfer perspective, the US will care too much even if he does go through with his threat.
The FAV operates Block 15 A/B-models that feature *degraded* technology from the 1970s. I suspect China is well beyond that time-frame in foreign technology exploitation terms. Give them to the Cubans and they won't be able to fly them (no spares) and probably wouldn't be able to do anything meaningful with anything they learned from them, either.
Politically, however, it's a different story...