It is most likely an issue with my records now that I look closer. He was certainly a 5.8k SG in season 19, but may have become a 5.1k SG at new seasons beginning and then switched to PF shortly thereafter, causing my confusion. The writeup done by a leaguemate at the beginning of season 20 claimed he was a 5.8k SG as well, but maybe he wrote it up before the update.
No need to look into this further, at any rate.