and once again...… what would happen if Canon did not allow third party lenses?
The store clerks tell the customer "Don't buy Canon, you can't use third party lenses with them".... and sales plummet......
You may get the odd sales clerk who does that, but I can't imagine most sales clerks would even mention it. The lack of third party EF-M lenses has hardly hurt that system.
Anyway, I've never had a Sigma lens I've been happy with. I wouldn't trust them to produce something that reliably focused on the RF mount.