Sure, they might do that. But they might also do that because of arms transports, transit points of foreign volunteers, and even sanctions (which they've already ridiculously claimed is a declaration of war). My point is just that the act of shooting down Nato planes outside of Nato territory (and the ocean) doesn't concern Article 5, no more than any of those other things. You might claim that's a literal reading, but there is no other way to read the article.
That doesn't mean a no fly zone is a good idea, of course. Whether or not Russians shooting down American pilots is going to start WW3, Americans shooting down Russian pilots probably is.