[throwawayallday]

OS/400 was developed alongside the AS/400. IBM introduced the AS/400 specifically as a "midrange system".

Source: me, two decades doing IBM sysprog on AS/400s and z/OS.

[lproven]

Yes, no argument there.

All I'd say is this: "midrange" is an IBM sales concept, and I don't think it was ever really used much outside IBM.

"Minicomputer" is a generally-accepted industry term, or at least, it was once. Minicomputers were not specific to IBM, just as mainframes weren't either.