It's not a good measure of coupling. Don't even think it's meant to be. Coupling would happen if these overloaded methods called back to their super class but they don't have to.
If they never called back to super and we're at 100% it would mean all the implementation in the super has gone unused, at least for this subtype.
All this really measures is how much implementation comes from the subtype vs the super. Not sure why I'd care about that.
Far more important is those calls back to super. You don't want to make your readers sea sick by sending them up and down the inheritance stack while they read code. This is called the yo yo problem and this stat does not measure it well.
The yo yo problem would be better measured with mouse clicks, or inheritance levels, or even the tears it brings to my eyes.
We tend to favor composition over inheritance today but you can make the same mess with composition. It's just not as likely. Rather then drag readers up and down give them a good place to stand and think about a small part of the problem while hiding as many unneeded details behind well named abstractions as you can.