“...pretty much the definition of a trust....”
In NO way is that the definition of a trust, and I defy you to prove otherwise. The only way anti-trust would have any bearing would be if the alternative advertising mediums coalesced to suppress X either by saying, “If you advertise with them you cannot advertise with us,” of if they slashed ad rates in a concerted fashion to undercut X’s rates.
Obviously this is ludicrous because (a) there are THOUSANDS of alternative mediums and (b) it isn’t an alternative medium that is being sued but rather the advertisers themselves.
This would be as if Bud Lite chose to sue not the people calling for its boycott but rather the customers who stopped buying their six-packs.
People campaigning for boycotts of ANY product are free to do so, and those who listen to them are free to stop buying the boycotted product.
To lazy to actually learn something yourself?
Look here, this page describes the Sherman Act, Clayton Act and the Federal Trade Commission Act from which I derive my much oversimplified but accurate summation of organizations acting as a trust to prevent competition or employ anti competition monopolistic practices.
So instead of posting uninformed baseless challenges actually go learn something.
In short it is not legal to create a body dedicated to denying fair competition to a business group or community. It literally does not matter if there are other alternatives, it is still illegal to try to create anti competition, go look up the anti trust laws and stop looking so silly with your uninformed assumptions.