Epic specifically brought several Sherman Act section 2 claims against Apple. The first step of proving a section 2 claim is establishing monopoly power.
So even though there are some antitrust violations (e.g. price fixing or bid rigging) where monopoly power is indeed irrelevant, it is very specifically relevant in the context of Epic's lawsuit against Apple.
Whether or not Apple has a monopoly is irrelevant for the antitrust laws.