They might be right, but the "Mini" was more like a return to the size of the 6 & 8; not the same size as the 5 or prior SE. So for me it was still too large.
https://imgur.com/a/iphone-mini-vs-iphone-5-vs-iphone-6-case...
The "usable screen" is where my thumb can reach, not whatever idea people have in their heads about the total size of the phone or anything, truthfully.
Anyway; hit recognition of the keyboard is so far behind where it was in the iPhone 4/5 generation that I doubt modern iOS would even be functional; even if you excused the padding issues that would inevitably be an issue.