This rationale is highly dubious. It seems extremely unlikely they would "wait" so long for this system without first putting in an existing system like the HHQ-16 which the PLAN seems to like just fine, had they wished for their carriers to be protected by a medium-ranged air defense missile. At worst they could have just saved space for this hypothetical new MRSAM system, but clearly CV-16 has the area paved over, and CV-17 doesn't even have such a space designed in at all. It is an unequivocal fact that the PLAN does NOT want an integrated medium-ranged air defense system for its carriers. Which leaves the possibility that they want such a level of protection for their lesser-value LHDs even less likely.
That is assuming at the time that they desired an MR SAM for CV-16 or 002 at the time at all.
My suggestion is that a new MR SAM system was finishing development and able to coincide with 075's design and construction timetable they decided to include it.
In this rationale, it is not so much that CV-16 and 002 "lack" an MR SAM capability when they were being overhauled and constructed respectively, but rather that 075 receives it as an "extra" because a new system completed development and would have been ready around the same time as it would've started construction.
Going forwards, it would mean that new construction ships like 003 would also be expected to have a VLS for the MR SAM because 003 would have also been drawn up and constructed when the new MR SAM was ready. CV-16 and 002 would eventually receive structural overhauls to accommodate a VLS for the MR SAM if it was feasible. But if it was not feasible or practical then they would simply continue with their existing weapons fit.
That said I'm not convinced 075 has a VLS yet, so the above is just discussion for a hypothetical for if it was present.
As for ASW missiles, the next best military mission for an LHD besides amphibious assault is ASW. This is a quite natural extension of the fact that it has the capacity for so many helicopters. Using escorts with TAS/VDS and/or dedicated ships with SURTASS-equivalents, an LHD with a large complement of ASW helos would be the premier ASW warship in the PLAN and a perfect escort for a carrier strike force when ASW is paramount, such as a confrontation against the USN on the high seas. It would function like the Hyuga/Izumo class, but even better. That new long range ASW missile with wings would be a prime candidate for loading into an LHD tasked with ASW.
As for LACMs, to say it's "unprecedented" is to not look at it the right way, since how many LHDs have VLS complexes that also have SRSAMs? None. The one or two that I can think of off hand that use VLS use them for air defense because they don't have SeaRAM or another equivalent SRSAM. So if a new PLAN LHD class does have a VLS along with the HHQ-10, all possibilities are on the table, including land attack, and medium-ranged air defense becomes much less likely. This makes sense, since shore and inland targets may need to be softened up before a landing, and bringing in organic longer-ranged land attack capabilities may be a good option in some or many cases.
I think ASW missiles are certainly much more likely than LACMs, and I can see 075 having an ASW role.
That said the 075 won't have the speed to keep up with carrier battle groups so I don't see them providing an ASW role for a CSG, but for various other mission profiles when appropriate, sure.
A case can certainly be made for giving LHDs an organic land attack capability, and I'm definitely not ruling it out as a possibility.
However I think it is far from the most likely payload for a VLS on 075 if there is a VLS, because giving amphibious assault ships like LHDs an organic in built land attack capability would be quite a deviation from worldwide norms and it means 075 would be the first of its kind in that regard.
Of course, if there were rumours that 075 was designed with VLS and intended to have organic LACMs as its payload then that would be an entirely different story.