The Master of the Ravenwing has always been mentioned as having a "special" Landspeeder variant with extra tech on it.
GW has just simply upgraded it to a Jetbike which isn't that far a stretch and makes him more unique...which he is meant to be.
They didn't say they had the STC for it, just that it's the last surviving example and is probably very well looked after.
Quote:
| POINT: don't let the fiction become more important than hobby |
That's a silly thing to say because otherwise people would just make their armies with items that are long lost super tech.
The fluff gives a tight constraint to stop this and to say he should disregard it is wrong.