Both the sellers for whom you have left negative feedback appear to have jumped on the antibac bandwagon, and both have a string of negative feedbacks.
I haven't even tried to keep abreast of all eBay's advice on the covid situation, but there's nothing on their money back guarantee policy page to say that these items are not covered. Someone else here will probably be able to tell you. Otherwise, it's possible the message was a fake.
You can open a case or cases through the resolution centre. The worry is that you already appear to have returned the items. If you had opened a case first, eBay would have required the sellers to send you prepaid return labels.
The big question is did you return these items by a tracked delivery service, or at least by a Royal Mail postage service that provides a delivery number on your proof of posting. Have you checked the tracking or delivery numbers on RM's track and trace web site? If they confirm delivery, you will be able to enter the numbers in the case, when eBay asks for them. Without proof of return delivery there will be no point in opening a case.
If these items are really not covered by eBay's MBG or you've left it too late, you can open a case under PayPal's buyer longer, 180 day protection policy.
It would have been so much easier, for you and for us, if you had read and followed eBay's rules or asked here for advice before returning these items.
