I don't think this is strictly true. They do tweak parts of the kernel such as the CPU scheduler to deal with new CPU designs that come out which have special scheduling requirements. That's actually happened quite a bit recently with AMD and Intel both offering CPUs with asymmetric processors with big and little cores, different clock speeds, different cache, sometimes even different instructions on different cores. They also added ReFS not long ago, which may have required some kernel work.

I can understand though if they have few experienced people and way more junior devs. It would probably explain a lot to be honest. A lot of Microsoft stuff is bloated and/or unreliable.