I think this got thrashed through on another thread. There's a risk of NCAA violations if there's a perception of coaching going on, including by former players. (It would be have to be done off campus, off UT's nickel, no encouragement by UT personnel to do this, etc etc.)
Dumb rule IMO, and maybe over-cautious on the part of current admin and staff, but apparently that's at least part of it.
--and no, I don't think that a pep talk by a former player should count as "coaching", but I don't even begin to pretend to understand NCAA rules. It was hair-raising enough having a kid recruited back in the day.