Average PAYE refund was €900 last year

I wonder does the average include just those in a refund position and exclude those in a nil/balanced position and others with underpayment?

That's exactly it. The source figures are quoted here.
Revenue said around 550,000 PAYE taxpayers have filed a return for last year.

Some 440,000 of these had overpaid tax, and collectively got refunds of €400m.

This works out an average of €909.

There will be a strong selection bias of people expecting refunds filing early in the year so expect that the proportion of people being in a refund position will drop a lot lower than 80% by the end of the year.