They may be ignoring it but they're not defying it. The constitution may say Congress has the sole power to declare war, but I don't think it says the nation has to declare war. We can simply not declare anything, and go do it. Technically that's not defying the constitution, it's just ignoring the spirit of the constitution.In other words, the President and Congress are (again) ignoring the Constitution.
This is also part of the magic of it. The presidents have never declared war. They're not allowed to "declare war". That's congress's job. We could get into the semantics of "declare war" and "declare" and the spirit of the constitution, but if you simply ignore all of that then there is no transgression. These presidents didn't declare these wars because they're not authorized to. Congress also did not declare them but the constitution doesn't say they actually have to, only that they're the only ones allowed to.President declares war without congressional approval, defying the Constitution.