How can you "distort customer choice" if you're only at 15% market share? I thought that's the whole point of using antitrust mechanisms against monopolies, but

How can you "distort customer choice" if you're only at 15% market share? I thought that's the whole point of using antitrust mechanisms against monopolies, but customers in monopolized markets literally don't have another choice. If Apple is only at 15% market share, it's pretty clear that people using Apple products are doing so because they prefer them to the (much more popular) main competitor.