This assumption is probably broadly right, but I wouldn't be surprised if there are a few (100+) supporters who are too half arsed to put them on the exchange. You only have to look round the stadium on match days where it is "sold out" to see that lots don't bother. I guess they don't need the money or decide too late not to go?
I think it's more that supporters who don't have tickets are reluctant to use the Exchange. I've put my seat on it twice and on neither occasion was it sold - but that's clearly not going to apply when there will be 20,000 S/T holders without a ticket.