That is true if I'm trying to find interpretation that is fitting to the original art. If I borrow movements from Goju-ryu kata, I won't be looking for Goju-ryu principles in them. I'll be looking for principles from what I know that work with the movements (as I see them).
You make a good point in this post, that's not entirely at odd with what I'm saying, FC. You and I are degrees apart, rather than at odds on this. If I tried to understand a Wing Chun form, for instance, I'd be crazy (and probably learning bad information) trying to understand it on my own. But if I use the basic movements (as I see them) to dig into principles from NGA, it'd just be an intellectual exercise that might be useful but is unlikely to cause harm. For instance, the stance (which name I've forgotten) that is throughout Siu Lim Tao is wrong for what I do - it simply wouldn't make sense. But there is a stance that's visually a bit similar, which is traditionally used when teaching a straight punch. It's close enough that I could substitute it. Some of the hand movements seem odd, but are close to movements I might use in hand-fighting or grip transitions, so I substitute the visually similar movement I know. In the end, my "Siu Lim Tao" isn't really Siu Lim Tao. It's an approximation using movements from what I actually know. And it makes me examine some transitions between movement that I might not choose to look at on my own.
Mind you, it's entirely an intellectual/physical exercise, not a real training exercise that would make sense in the long term. It'd be something to tinker with and get me thinking differently. A bit like when I go to a seminar in a style I don't have experience in. I won't really learn those techniques properly (as defined by the art in question), either, but I pretty much always come away with a new thought or understanding about one of the arts or techniques I already knew.