I think a vow of this sort is a very personal one, so I'd guess the rules are sort of up to him, it sounds like he stopped speaking, but communicated in other ways. The article mentions he earned his PhD during his 'vow' so I would guess he'd have done at least a bit of writing in addition to mail.

His vow ended in 1990, so no texting, and likely no email.

He completed a doctorate, so clearly he wrote manuscripts / his dissertation, but I'd have to imagine that he regularly used written communication with his advisor. Probably lots of letters, overall.