I don't think that all these available are seat exchange. Yesterday rows of seats appeared towards the top of some of the 3rd tier blocks of the Colin Bell and East stands. The same happened before the Leicester game. I'm guessing they're unsold corporate returns.
There are over a thousand available at the moment.
It's probably worth remembering that not all of the available seat exchange tickets will translate to empty seats. If a ticket doesn't sell then then the seat owner will probably transfer it to friends and family for free.