I disagree, but I think this is a question that could only be answered in court (and probably over several court cases).

The end-stage result of your argument would be that copyleft licenses are practically unenforceable because if you just make a separate textual file and then make reference to internal APIs in order to make extensions then you're simply using an interface regardless of the implementation and thus not bound by the license. So the strongest copyleft license you could possibly have is an even weaker form of the MPL ("function-based" rather than "file-based", and one that you could trivially bypass).

As for GPL condoms -- personally, I think a judge would not look favourably on GPL condoms precisely because they look like pedantic rules lawyering. If usage without a GPL condom would be a violation then I would be surprised if a GPL condom made a lick of difference in the eyes of a judge. But I've been surprised plenty of times, and as far as I know they've never been tested in court.