"sa" seems too short to alias, it can result in people making typos and adding it to posts on accident.
In fact I think this has already happened, as the 5 posts under sa seem to be from five different artists:
post #689455 is from pixiv user sa1931
post #913064 is from pixiv user sa0418
post #1215776 is from pixiv user smas5378
post #1614306 is from pixiv user netwavers
post #1809031 is from an unknown twitter artist