The landfall ability checks for an action that has happened in the past. It doesn't matter if a land that entered the battlefield under your control previously in the turn is still on the battlefield, is still under your control, or is still a land.

The effect of this spell's landfall ability replaces its normal effect. If you had a land enter under your control this turn, only the landfall-based effect happens.

Once the spell resolves, having a land enter under your control provides no further benefit.

Having more than one land enter the battlefield under your control this turn provides no additional benefit.

Whether you had a land enter the battlefield under your control this turn is checked as this spell resolves, not as you cast it.