It's a decision they've made. Perfectly reasonable. If they did accept AI code, that's perfectly reasonable too. They just made their policy clear here..
¹: Often correlating with a marked decline in the quality of their technical writing. I don't know which direction the arrow of causality goes, but I have observed LLM use inducing madness under reasonably-controlled circumstances – different phenomenon, similar principle. I suspect the use of AI codegen systems is causing the reduction in discernment ability, rather than a sudden drop in discernment ability causing increased use of AI codegen systems.