Two different questions. For the record, while I'm not an expert on records retention for government funded universities, I do have two roles in my professional life.... one is operational risk management and the other is records management.
If the messages are on a personal device it is not illegal to destroy them. BUT, the university should have and enforce a policy that university business should not be conducted on personal devices. Its probably not as regulated in that space. I work in financial services and if you are conducting business with a client on a personal device and get caught, you're fired 100% of the time because you just put the firm in a regulatory non-compliance position.
For legal holds, it is irrelevant. Legal holds are to retain records that you OTHERWISE could destroy. So say I have 10 year old account statements for a client. I can destroy those because they're older than 7 years. UNLESS that client is suing us... then I can't. So regular retention requirements apply and then legal holds trump expiration of records.
*I am not a lawyer and I do not have specific expertise in government/university records management.
The entire second part of your post is an excellent question. I think the committee owes the public an explanation. Did they try to recover the texts and were unable to or did they simply not even try. Could the cellular carrier provide copies?