OpenAI quotes from iMessage communications between Liu and Peng But Apple's motion states that their communications occurred "through the Line messenger app to avoid detection by Apple" https://storage.courtlistener.com/recap/gov.uscourts.cand.47...
It also does specifically state that they use that to avoid detection (and only between those two people) so they could have been using multiple communication mediums.
That document has a lot of more interesting stuff, however. They also talk about an "apple proprietary metal finishing process" made by a supplier - could they have tried to steal Liquid Metal?
edit: They also state this below which may have some more specifics on what technology is in question:
"Documents sufficient to show hardware development at OpenAI involving (1) bat- tery technology, including can size, flange specifications, and manufacturing toler- ances; (2) the metal finishing process, including surface preparation and anodization"