I say this only partly tongue-in-cheek, but I don't think the smell test applies to the US. If it did, the Zumwalt wouldn't have survived the first design meeting.
Anyway, I see what you're getting at that this requires a significant degree of trust. I think that trust is there between China and Russia and will remain so for the foreseeable future (for at least as long as the J-20 remains sensitive technology and probably long after it's become obsolete). I would also agree that even given that trust the PLAAF would still want to hold its cards close to its chest, except for one thing: I think it also wants to show off. In fact, I think it's dying to do it.
Think about it, since 1949 until the Sino-Soviet split, the USSR was the richer, bigger, stronger, and better-looking older brother. Even after the split, Russia remained the dominant power in the Communist bloc - the admiration (or envy) was still there, it's just that added to it there was now acrimony.
Decades later and fortunes have changed. China is now the one, as the kids say, ballin'. You bet it wants to invite Russia over to the new house to get a look at all the new toys and drive the Lambo around! This kind of flex is a dream come true for all the old-timer generals in the PLA. That's why we hear reports of Russian soldiers getting up close and personal with all the new gear and the J-20 participating.