Maybe the US sent their F-35(s) to this area knowing it will prompt a response from J-20s. I suppose both sides got valuable insight about each other's capabilities in these encounters.Wuhu not longer has any Su-30MKKs left. If they were the closest base to the action then they have to send what they've got. Also, if situation does get intense (very unlikely but possible), would you rather confront an F-35 in a Flanker or a J-20? For routine intercept missions it is also possible to mask your signature with luneburg lens, provided that you fly higher than hostile AWACs.
Also keep in mind that J-20 has entered mass production phase and even non-elite frontline units will start fielding them. Stealth on stealth intercepts will only get more common as legacy fighter aircraft get phased out.