Author of the blog post here. That explanation sounds very plausible to me!

If the whole enclosing function became inlinable after the reflective call path disappeared, that could explain why the end-to-end speedup under load was even larger than the isolated microbench.

I admit that I don't understand the JIT optimization deeply enough to say that confidently... as I mentioned in the blog post, I was quite flummoxed by the results. I’d genuinely love to learn more.