For one to use anti monopoly law, being monopoly is kind of important. If you are trying to lock in vendor while NOT being a (near) monopoly - it is not illegal per that law. There might be other regulations, that makes it illegal though, I am not a layer to know this fully. I am just conveying somebody else’s analysis who is a lawyer.
I stand corrected, while android dominates the world, in US iPhone share is indeed just above the half (55%). It is still not enough to call it monopoly, but it might have chance.