Vader47000

Correction: Also, present Bill and Ted hear their future selves introduce him as Rufus. Thus their learning his name is part of the time loop.

Vader47000

Correction: He doesn't tell them his name on screen, but they spend a lot of time together off screen. He could easily have told them without it being shown.

Rufus never told Bill and Ted his name when they first meet him so there's no way future Bill and Ted would have known Rufus' name too.