Definitely not an unexpected development, though I'd wonder how such a weapon would be able to achieve 500km range while also fitting in four within the J-20's belly weapons bay, as well as the size of its warhead.
Still, having such a weapon for stand off missions -- that will likely be capable of striking both anti ship and anti surface targets -- will be a great enhancement to both J-20 and the future carrierborne 5th generation fighter.
I imagine a large number of this missile will be able to be carried by H-20 as well.
The volumes of these missiles will be greater than a typical cylindrical AAM as pb said they were akin to rectangular prisms. It'll probably look like a smaller KEPD 350. Missiles like the PL-12 and PL-15 have fixed fins that require significant offset leading to wasted storage volume. A rectangular missile would be able to pack pretty tightly in the weapons bay, leading to something like 80% or more volume utilisation.
One half of the J-20's weapons bay is around 1m wide and 4m long, so two tightly-packed rectangular missiles could have dimensions of ~40cm wide and tall, and ~3.5m long. That's a decently sized missile, around that of the NSM. With long fold-out fins, which it almost definitely will have, it could fly quite a distance.
