Not necessarily. I have found, close to departure date, when only a couple of Y (literal Y, not B, K, S etc.) WT/ET seats were left that there was a redemption seat available. It wasn't a GGL redemption or Gold extra availability. It was a simple redemption seat.

Avios use in WT/ET can be excellent value under certain circumstances.

This one saved me a large sum of money because it was a flight I needed to book, rather than I merely wanted to book, and I'd have paid the full Y for it if a redemption seat wasn't available. (Nothing was available in any other cabin.)

P.S. I wasn't the traveller.