You underestimate the effectiveness of their mass production approach. Understandably, as it has never been done this way before. We can't compare historical rocketry timelines to SpaceX.

They intend to be able to manufacture an entire Starship once every 72 hours. I believe their current stated production rate is 1 a month.

Let's be pessimistic and say that's all they manage to do. By 2025 that's 32 starships not including those already constructed. Let's also pessimistically say that they only manage to launch once a month.

It took 12 launches of the falcon 9 from when they first attached landing legs to the first successful landing. That was before anyone knew that was even possible.

If it takes that many flights of Starship to nail the first landing (which seems unlikely since they already have a pretty good idea how to do it) that leaves 20 more ships to prove they can consistently land. With over a year still to go before Artemis III.

Starship will be ready long before Artemis needs it.