I do think it's kind of both. "Sys" and "admin" are accepted abbreviations for "systems" and "administrator" obviously. "Sysadmin" is a recognized portmanteau of the two abbreviations, agreed. But "sys admin," non-combined is obviously still two valid abbreviations that make sense.
It's like arguing "sex expert" isn't a valid term because "sexpert" exists; they're both valid ways of expressing the same thing.
