There's no direct evidence from SOD, but after reading the scroll use rules, it does appear that Xykon would still be able to use scrolls.
I think we're getting a bit too nitpicky, though, and although being too nitpicky is arguably the entire purpose of this thread, in this case I don't think it would serve the purpose of nailing down character info very well, since the way Xykon and Redcloak collaborated on making the former a lich was obviously houseruled and probably wouldn't stand up to strict scrutiny. I think we should just stick with listing the basic requirements that lichdom requires of Xykon and leave it at that.