If you go ahead and do the updates that do show up as available, there is one called "mintsources". I think that must fix the situation, as I did not have to manually remove the offending sources from my list. They just automagically disappeared.
Interesting. when I first tried Mint KDE, around a year or so ago, that was the first update, I think, and it made my machine hang. I had to seek help and I think ended up unlocking something and retrying and it fixed it. This time around it just installed fine, but clearly doesn't seem to have fixed this problem. Unless it was "mintupdate" I'm talking about. Whatever, I do regular updates and always take whatever's there.