Correct… although it is refundable if they end up sitting someone there.
In my friend’s case there was a good chance the seat beside them would have remained free… but… given it was a 2-4-2 airbus, some bright spark could have specifically wanted a window or aisle seat and just happened to select the one beside them!
So this gave peace of mind that the seat would, bar any major problems, remain free.
It’s probably more relevant for folk without status? Although that’s an interesting question… if the flight is sort of getting close to full… do free seats go as shadows for plats, or to the people who pay?