Not cleaning off old glue was the norm back in the day. If the glue didn't pull off with the tire's removal it was pretty well bonded with the rim, just what you want a sew up glue to be. Also some sew up tape instructed the installer to use a base layer of glue before the tape was used. I don't remember what the various Tufo tapes instructed users to do any more. Maybe you can search for an instruction sheet on line to find out, or visit a LBS that stocks Tufo tape and read the included instructions
Either way I don't think it maters if old glue is still there as long as it's not so thick as to cause a lumpy tire seating. For what it's worth the many tens of thousands of miles (and dozens of tires) I rode on sew ups I only used rim tape as an on the road flat replacement, a temporary situation as the spare tire was removed and the tire (new or spare) was glued as usual.
this isn't a bad question as rolling a tire is always a nasty thing (I can still hear the sound that a tireless rim makes on pavement as it slides out
) Andy