The subpoena says that a personal appearance is necessary.
In lieu of an appearance, the custodian may fill out and return the declaration enclosed herewith along with the requested
records.
He is not being asked to testify regarding evidence, all he has to do is certify (at the penalty of contempt or worse) that what he has submitted is true and proper.
I am certain that he would not be able to per-record his testimony as this would deprive Ross his right to face his accusers and to question theymos if he wanted to do so. In theory it could be done via skype or via some other video conferencing software, however this is generally rare.
If he was being asked to testify, of course they can do it before hand. The plaintiffs do one deposition and submit the logs to the court and defendants, and then the defendants get to do a similar process and are free to cross examine anything that is said. I think you might have been watching too many films, but a lot of what happens in the court room is processing and reviewing evidence that has already been submitted in documents. It would be rarer that new evidence is freshly submitted in a court room as no one can do anything until both parties have had time to review the evidence and prepare a response.