I wouldn’t be so sure they are not launchers. They look like an evolutionary developed of the HQ12 launcher. But of course they are not meant to be launchers for the missiles they are displaying for the parade.
I would say they are most likely repurposed for the parade, with parts taken off to allow them to serve as display carriers, as opposed to parade-specific vehicles that serve no other purpose. A subtle but important distinction, especially in context of how ridiculously tight the PLA land forces have always been with money.
Whether they are using launchers similar to vehicles for past systems is irrelevant imo -- Neurosmith was stating if those vehicles would be able to launch the missiles they're carrying and thus allow us to extrapolate what their actual launch system is meant to be (UVLS and/or others). And as you agree, the vehicles here are not launchers for the missiles they're displaying.
For what it's worth, we have seen past display trucks for missile mockups standalone which are very similar in geometry (not identical) to the display trucks that YJ-15/17/19/20 are on, for example HQ-9, YJ-83, YJ-18 and YJ-12. And we know what the actual launch systems look like (YJ-12 ground based TEL and slant launch canister, YJ-18 from UVLS), and they have not resembled those display trucks at all.
We've even had display trucks for dedicated naval systems like HQ-10, which obviously is not intended to have any such launch apparatus as a naval system aboard ships.
Considering the amount of money used for these parades overall, I wouldn't be surprised if they are able to comfortably spend some money to take some utility trucks to be fit out for display purposes. But chances are they did not refit trucks whose normal function are launchers.