who's ahjussi (TДT) is that the semes name?

Yes

It’s a Korean word for someone who’s an older man :)))

It kinda means uncle, there’s similar terms in practically every Asian language. It’s used to refer to males who are older than you and are middle aged or anyone who would seem like an uncle

The seme's real name is Bum Geon Woo. It was stated on the ID Eui Joon found