When there is corroboration from both sides, it is very likely to be true. J-20 pilots have claimed that they have encountered and intercepted foreign aircraft and now it has been confirmed by USAF general.
The Indian claim was a classic case of misreported information. IAF never came out and stated that they've detected the J-20. What actually happened was that B.S. Dhanoa came out and claimed that J-20 is not stealth and that MKIs could easily spot it from "several kilometers away" (which is technically not wrong
) and somehow it morphed into Su-30MKIs have spotted J-20s at 150KM range in Tibet. Never mind that MKI's radar has a range of 110KM against typical fighter sized target.