I am one of the concerned simple users.
I am not a coder but I tried to do a search in the extension code for these parts, although I can still not be 100% sure what it does and I also don't really want to dig code whenever weird permission requests pop up.
While I hope you don't take it personal I am sure you can see the irony in the "don't worry" message nowadays, the one thing common in a lone dev, a company or a governmental organization is that such a message doesn't really offer any assurances from anyone.
I personally don't really understand this icon badge count (although I only use autofill for 1 site, 1 rule, so I don't know what count would show where, perhaps on the icon itself) but it doesn't seem like a feature that would be required for the core functionality. It may be useful but probably not necessary for the autofill function which would mean in my view that the minimum required permission principle is not applied.
It is perhaps not the biggest concern with google's permission management system that we have which is still pretty great for exploiting personal data, but this situation was something I cared for enough to post my point of view here, as a self-proclaimed representative of the stupid users that are bothered by this but generally don't come here :)
I personally don't require further explanation, it would also probably be wasted time so I don't need to be "convinced", I just posted here because I think it would be great for a lot of people if some different solution would be used, if privacy is a concern of course. I don't know how many people actually care so I don't know if it is "worth it", that's a determination you'll need to make.
thanks