Yes, Apple provide both the OS and the DAW. They don't however, provide NI plugs or have anything to do with NI coding.
For example, I don't have any NI plugs but I do have iZotope and Waves. Some Stephen Slate stuff… Would I expect Apple to test all of iZotope, Waves and Stephen Slate code before it issued an OS update? Of course not. I'd do exactly what Apple did and that is provide the code, in full, as Apple itself was developing it. They did this not only for the developers but, for the first time ever, the general public too. This way the developers can alter their code and issue updates to their users if any new Apple code affects their own.
If Apple had to check all the code for all the current *possible* plug designers from 3rd party developers there would never ever be an update issued because it would be impossible.
If Logic itself had a massive show stopping bug it would be different because Apple make, as you rightly say, both the OS and the DAW.