[flagged]
It has been eight years since Obama's presidency, had there been any use of this hardened BlackBerry for classified communications it would have emerged by now. Similarly, all messages on that device were subject to the Presidential Records Act, and are archived by NARA. You can FOIA them if you want to.
There were also no claims made during his administration that he ignored security protocols. Even his insistence on retaining a BlackBerry for unclassified communications was done through a compromise and an NSA-hardened device, not by ignoring the rules.
Similarly, how do we know that Reagan didn't hold cleartext phone calls with his aides on the Top Secret plans to contain the USSR? We don't, but in the absence of any supportive evidence over the years it's safe to assume he did not.
(I would have replied to him directly, but the comments have since been [appropriately] flagged)
In reality, no argument could ever be made if you had to prove the negative of every argument. Some other common applications of this fallacy off the top of my head:
"Well we don't have proof that children weren't trafficked in Comet Pizza, so it's proof that it did actually happen."
"We don't have proof that no kids used litterboxes at school, so it's proof that they did use litterboxes."