I disagree with this.
If you are riding the face of the wave then you are riding waves, regardless if it's onshore or not. In good sized windswell you can let your kite drift completely and be 100% powered by the wave your are surfing.
The main issue with surfing onshore is the wind speed vs. the wave speed as once you are heading downwind with the wave the kite's relative wind drops substantially.
One way to get around this is high wind conditions, when the wind is 25+ knots and the wave speed only reduces that to 17+ knots you are fine.
In 20 knots of wind you'll be down to 12 knots at the kite, and at that point many kites will lose responsiveness and may even drop.
Personally most of the riding I do is riding onshore windswell and I value two things:
1) the ability for a kite to drift in as low a 8 knots relative wind
2) upwind ability of the kite because riding onshore means long tack chains to get upwind then zooming downwind riding the wave
The best drifting kites for onshore are lightweight single strut kites that luff when depowered allowing them to create downwind drag and move downwind with you.