You're all missing the main point. PL-15 and PL-17 do not diverge from conventional air to air missile configurations. Okay PL-17 is very unique with no canards for a long range AAM. But now we know PL-16 is supposed to be thinner than PL-15 and offer greater range on the purely physical parameters. In the mid to late 2010s when PL-15 and PL-17 were revealed, we had little real indication of range and performance parameters. We have a much better idea of their range and performance parameters now.
The latest hints/confirmations for these missiles that were not known until 2020+
- PL-15 has between 200km and 300km range (depending on so many factors obviously)
- PL-15 uses AESA seeker
- PL-17 is hinted to be 700km max range against large, slow targets and 400km against fighters
- PL-17 is designed to also target fighters
That's 5 - 10 years of them not knowing and not being able to put the physical look with the performance.
Now their size and observable characteristics can be considered together to interpolate other information that potentially can be used to counter them. There's no good reason to reveal the design and shape of your AAMs because these things are pretty much going to be operating on such universally converging designs and technological limits. It reveals too much about your unknown factors such as energy density of fuel and the performance of your rocket motors.
PL-16 and PL-17's successor or rather, any other AAM will/should probably be kept hidden until their successors have passed trials and close to reaching service. Especially if the performance parameters have been leaked ie PL-16 has greater operational windows (range and other attributes) than PL-15 while being thinner. They can put 2 and 2 together and interpolate too much from its physical characteristics that can be exposed by photos.
Missiles are top secret. China plays by this golden rule. Revealing stuff that is "safe" enough to reveal if it's a novel and strategically important missile but revealed for various reasons (part of which is deterrence) and revealing stuff that have become superseded. Missiles like YJ-17 and YJ-19 fall into the former category. Missiles like DF-17 and JL-1 fall into the latter category (DF-27, CJ-1000 etc presenting as superiors).