I wonder why he didn't use AOT compiling if he's worried about JIT warmup.

My guess is because .NET AoT is not yet optimized and mature enough as JIT. This is known and is on the agenda of Microsoft but it will take time to get there.

It wasn't discussed, so we're left to speculate. If I had to guess, I imagine that the .NET JIT has actual benefits: the variety of architectures has gotten enormous and JIT is likely a performance win after warmup.