I don't think anyone has stated that the J-20 has cleared the prototype stage. It's only been test flying for half a year. I doubt anyone think it's fulfilled its requirements either.
yeah but i meant for example
The Rafale A was fitted with the F-404 but the later Rafale C and M used the french engine.
Similar could be the J-20, it might use its WS-? and later the intended engines, the Rafale A was a proof of concept.
The EAP was more different, it was powered by Tornado engines but most of it was a Eurofighter.
The YF-22 carried the core engine F-119 and most of the basic configuration, they were tech demostrator not research aircraft.
the X-29 was a research aircraft and same was the X-31 but the EAP was, i do not see why people think is offensive to say the J-20 is a tech demostrator that might be refined and change like the EAP turned into the Eurofighter.