In fact, he was a one-time ally, later turned Legion foe in his second and final appearance...which was the previous issue before the Computo story began, so all that would have been fresh in readers' minds.
Although...hmm...now that you mention it, this raises the possibility that since Beast Boy died right after fighting the Legion, they may indeed have been the ones responsible for that memorial. I may have been too hasty in saying none of the other memorials were written by Legionnaires.
(And yes, in old school LSH fandom you'd better believe we spent months discussing stuff like this. In agonizing detail. Is it any wonder we all had to be institutionalized?)