They are following the FDA guidance for the convalescent plasma EUA, which does allow people that were sick prior to vaccination to donate plasma...it seems the Red Cross isn't interested in trying to communicate that to potential donors.
https://www.fda.gov/media/136798/download
If you expand the boxes on your link, you'll also see that they aren't specifically collecting convalescent plasma, as demand has decreased. Perhaps that is changing with the recent uptick (but given the demographics of blood donation, they'll probably have to change their rules if they need a lot of it; most people donate ~1 time).
