Wikipedia indicates that Wirrick was Sylvester's guitarist and co-writer by 1976, i.e. before even the first album by Sylvester. It seems that by the time of the 'Call Me' album, Wirrick was doing a whole lot of songwriting (together with Jeff Mehl) and production. So I would guess he indeed continued his guitar duty too.

P.S. Also of note that according to Discogs, the credits straight up list Wirrick as 'all instruments performed by' (apart from some additional stuff on individual songs), which is quite impressive.

A minor correction: it turns out that you refer not to the album version but the remix. This is the Discogs page for all releases of the remix, and the credits list four guys having made it. However, the guitar parts you mention are present in the original too, though seemingly less pronounced (if the Youtube upload is accurate).

Btw, if you dig Sylvester, check out Patrick Cowley, his friend and collaborator. Sylvester signed up on Cowley's and Marty Blecman's Megatone Records label after having decided to change direction from disco in '82. I'm not into hi-NRG myself, but I like the compilation of his tunes used for gay porn and slowed down by John Coletti, the producer of said porn: ‘School Daze’, released by Dark Entries.