I don’t think his comment was dismissive of the idea in general. The current form of the patch isn’t really possible to review; a single giant patch with no discussion whatsoever.
This is a big enough change, that affects enough of the internals of the std library, how easy it is to port, and would cause any other WIP branches to require manual merging so would need some timing and coordination to land, that an RFC does seem reasonable. Basically, something discussing why this move is a good idea, what porting use cases it would help out, if there are any alternatives, etc.
In addition to that, once the RFC process is done, I think this would be better done split up into a series of patches; one huge patch makes it hard to break down and verify that each move is sound and doesn’t change anything important; particular concerns might be privacy, error reporting, and the like.
I would not take his response as dismissive; just saying that once seeing the change, it’s clear that it will require a little more discussion before it’s reviewable and mergeable.