I just took a crawl through the kalendar of the Sarum Breviary (I know—a modern one…). Of the confessors listed, there are precisely two who are not listed as either bishops,  archbishops, popes or abbots:

• Petrocii Conf. (June 4th) concerning whom I’ve been able to find no data (and who isn’t in the Warren edition of the Missal).
• Translatio S. Edwardi regis et confessoris, inferius duplex, ix. lectiones. (Oct 13) Royalty—not clergy.

Jerome is an odd case. He’s listed as: Hieronymi presbyteri et doctoris, festum inferius duplex, ix lectiones. He’s the only “doctor” present so theoretically he ought to be considered a Confessor as well.

There’s only one saint in the Sarum kalendar designated as “presbyteri” with no other qualifications and that’s the memorial of Eusebii presbyteri. (Aug 14) whose status is questionable. This Eusebius was a priest of Rome who may be a confessor or may be a martyr—it’s unclear. In any case, he is the only “presbyterus” in the Sarum kalendar who’s not a martyr.

So—while in theory the Confessor category included all non-apostle/martyr/virgin/monks, functionally speaking it was for bishops and abbots.