I agree with the assertion that there's no proof of a full killswitch based on known past events, but the above quoted statement is also a lot more definitive than I'm willing to be.

With a fighter jet as dependent upon electronic support systems as the F35 and which is sold around the world why wouldn't you put a highly classified backdoor killswitch into it just in case?

The idea that such a killswitch might exist is one that could have always reasonably been pondered, what's new is any/all non-US "Western" governments having to seriously entertain the idea that they would end up in a situation where the US would have a reason to use it against them.

By what mechanism is this mediated? Because that sounds awfully similar to a kill switch in terms of the end result. Analogy by way of enterprise software: "We didn't remotely disable the software you purchased from us. Rather our server simply refuses to service your requests which happen to be required for the software to function." (Evil laugh from man with goatee immediately follows this statement obviously.)

And you know this because you've personally audited those planes?