I'm not a Sherman Act expert, but the language used in the Ninth Circuit Court of Appeals in the opinion upheld by SCOTUS in Alston has a pretty good explanation:
"The treatment of Student-Athletes is not the result of free market competition. To the contrary, it is the result of a cartel of buyers acting in concert to artificially depress the price that sellers could otherwise receive for their services. Our antitrust laws were originally meant to prohibit exactly this sort of distortion."
If the portal rules are structured to restrict free market movement to the point it prevents a player from earning money, especially more money, in a given year I think it may violate the Act. I think instead of making transfer rules more restrictive, you're actually going to see it become less so to the point of elimination of any sit out rules. Everyone will have immediate eligibility. I'm at a loss as to why it hasn't happened already and the NCAA's piss poor explanations when they grant or deny immediate eligibility aren't helping.