Exactly. If you're not a monopoly (or otherwise massively dominant in the market you compete in), antitrust law doesn't apply to you. Antitrust law is specifically to guard against the situation where one player in a market is so dominant, that the market itself becomes less competitive.

Although usually when I hear people misuse antitrust concepts on tech forums, they're not trying to understand complicated legal conflicts, rather they're grasping at straws to use as ammunition in dumb flamewars about platforms/companies/etc. that they don't like.