owner had a reputation for looping his own Piper Navajo
Indeed, casual aerobatics in aircraft clearly not intended for aerobatic flight is very foolish. The lesson obviously is just don't.
In the case of the Navajo (and I know nothing of this event, other than the referenced report) though I agree that it would appear obvious that the aircraft was dramatically oversped, It is much more likely that this was a result of a botched roll than loop. (And this is intended as the warning part of the message here).
A botched roll or wingover is the fastest way to get in trouble with an aircraft which will build up speed quickly, and this would seem to prove that. A loop is a bit of a different maneuver, and actually a bit more resistant to overspeeding, but also not appropriate in a Navajo (if even possible).
It is a mixed blessing to our industry that amazingly skilled pilots like Bob Hoover demonstrate aerobatics in non aerobatic types. It's great to see the skill, but it creates an "I can do that too" attitude, which is very likely unfounded for everyone else.
There are a very few special pilots, who can achieve these maneuvers with a margin of safety and thus success. Everyone else should not try!