It seems incredibly hard for people to grasp how antitrust law works. It's very simple: Antitrust law only affects those with a massively dominant control over a market.

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.

No. I think it doesn't matter to them until it becomes the majority platform (not that that's any less stupid).