If they achieve escape velocity, then they won't be hanging around the moon.

Sure, the article doesn't say that something would be hanging around the moon permanently, it evaluates the damage that might occur if the orbiting spacecraft passes through the splash of debris ('the ejecta sheet') as it's happening - it doesn't really attempt to evaluate how likely it is to hit it, the discussion is about the expected consequences if it happens.

Although it does assert that the NASA Gateway orbit passing through the ejecta sheet "will probably be several times before the sheet is dispersed."

Yes, that's why I said "only in the immediate aftermath of a landing".

The problem is, your orbiter is necessarily in orbit at the same time that your lander is making its landing. And the lander kicks up debris that can then threaten the orbiter.