That‘s indeed the point and at least I have no answer yet …
Just out of memory, we saw allegedly a J-15B under construction in February 2022 (or was it even 2021?) and soon thereafter first „clearer“ images appeared suggesting, it may have entered production.
However - and similar to the „missing„ J-20 units I‘m quite sure
@Blitzo does not agree with me - I think if there are already more out and maybe even in use preparing for service and teaching pilots, we „should“ have seen more of them. Even more so, we should have seen at least some with PLA logo and not only in grey with the shark on its tail, which IMO hints it is still a prototype or pre-serial.
As such, if flight testing off the Fujian should start somewhere in 2024, then at least more J-15Bs must fly already off land bases, pilots and technicians must prepare for the next phase … but where are they!?
As such, either I‘m again missing something or - as proposed by others - the PLA‘s OPSEC is just that good.