I just read a Fox News article that said it was specifically her machine, so it looks like (if the article is accurate) they are trying to say local mail via an email client. Before we switched to Google govt, all my email was in Tbird, but via IMAP (a requirement). I could locally archive all the mail I wanted and remove it from the IMAP server to clear up space, but my deleted mail was still archived on backup servers somewhere in DC (I don't know for how long or if forever). I can't believe that the IRS wouldn't also require IMAP for email clients.
The "her computer" narrative does give them the option of throwing her under the bus and laying all the blame on her, saying that she maliciously deleted emails. At least that's what the IRS bureaucrats might be thinking, because it appears they didn't contact any of their IT folks for input before they threw the "dog ate my homework" line out there.