I think he was on the defensive because you said:
an empty editbox should be just as much a "cancel" action as Cancel. if you accept the user inputting empty data as actual data then your UI sucks. If you're trying to implement the ability for a user to "clear" a variable then you should have a separate "clear" or "delete" button in your UI.
So first, his design sucks if he doesn't take $null as a cancel action, but then you give a very valid example that use $false NOT as a cancel action, so $null not taken as cancel = sucky, but $false not taken as cancel = fine, why would one not use $null as the same bool value as $false in your very valid example?
While I'm at it, you suggested to make a dialog in this case, but $input handles a number of feature that nobody is going to reimplement just because $input's design doesn't allow one to handle all situations.
