From what I can find M2 is ARMv8.5A whereas the A15 uses the slightly older ARMv8.4A ISA.
I don't know how much practical difference there is between the two, but supposed the M2 has for support for any instructions added in the 8.5 update. The ARM blog page on what v8.5 added doesn't make it seem like a major overhaul. Some instructions for better random numbers and a few things to make it harder for the chip to be exploited by certain types of attacks, but nothing earth shattering.
Making it harder to exploit is something Apple would want, but it is possible they added that stuff to the A15 core and updated the CPU ID to say 8.5A as a result without actually using the A16 core.
I'd want to see multiple things A16 has which A15 does not - which may be difficult as A16 is barely changed from A15. Because there are so few changes it is all academic, but I would find it noteworthy if they took the A16 core and backported it from N4 to the N5P process M2 was made in.