That's not strictly true tho. NOT all equipment shown are in serial or even IOC but only in service which could mean undergoing OT&E at FTTB units. What is true is that all equipment shown have been delivered to the end user(aka the PLA) hence in service. GJ-11 was stuck in OT&E hell for years as I recall and I still don't think there are any GJ-11s deployed in combat units yet. But if things go well with these drones we could see live units at Zhuhai next year maybe even a flyby.
I think GJ-11 is the exception. Every single weapon shown in past parades are in service in some numbers. Also in terms of preventing others from spying, it makes zero sense to show off weapons that are cutting edge in a parade. Cause your opponent can copy you and match your capability.
Considering how PLA operates, they will only show capabilities that are mature and also less strategically significant so that showing them off will not cause a big loss in terms of your opponents matching you.
I actually believe PLA has even stronger tech that are secretly in development and most likely in prototype stage similar to 6th gen planes. In fact they could even be in active service but not shown.