>Because the calls were not between that person, they were between two other people

I think one of us is misunderstanding the tweet here.

I read it as these calls are between OP and OP's Partner's mother. Meaning OP is a first-party in all of the calls. OP may not have been the source of the data, but OP was involved in the data, so the data was revealed to OP when he exported all of it.