The courts can frown on deliberately choosing mediums that can’t be subpoenaed. For example Musk didn’t provide a screenshot of a Signal he sent; the recipient also received a subpoena and sent a screenshot of the text. Twitter is therefore asking for adverse inference about Musk’s discovery compliance.

But also notice all the times they ask for a phone call, which wouldn’t be recorded (other than that it happened and length). Of course the contents of that call can be asked about in deposition but people lie, misremember, don’t recall, or “don’t recall” all the time in depositions.

Surveilled doesn't come into play here. The text messages were ordered to be disclosed and the people involved complied. It doesn't matter that the conversation was encrypted, if you have a record, you have to turn it over. Using a messaging service that automatically deletes old messages might be a way to avoid it.