At that time, all corporate email was already being hosted on an internal version of Gmail, so yes, both were technically possible, and external Gmail accounts

At that time, all corporate email was already being hosted on an internal version of Gmail, so yes, both were technically possible, and external Gmail accounts were in fact backed up. To do so would have rendered any instructions not to delete any emails moot and unnecessary. Evidently they decided not to do so, and instead, tell everyone to NOT delete any sensitive emails. One can only speculate why that might have been.