From my non-lawyer understanding: the Sherman Antitrust Act and any further bills/amendments aren't exactly explicit on what it means to be "anticompetitive" because it depends on the circumstances, and the writers wanted to allow the definition to adapt to changing circumstances. Regardless of if they buy them all at once, or buy and shutting them down serially, it's still arguably "anticompetitive". It's ultimately up to the courts to decide (if the FTC presses charges).