"A choice isn't a failure."

Oh dear...and you're the guy who bandies "sophistry" about as a criticism too...?

Your claim was "Turnout"

...Which is exactly what they didn't do. Whether they "chose" not to turn out or otherwise is irrelevant: some may, some may not have, but 40% failed to do so.

A 40% "non turnout" can't be made into a 40% "turnout".

As above, you're dealing in fantasy: filling in a lacunae with imaginary content.