I don't see Charlie mixing up a Seamus Finnigan claim with a Dean Thomas claim as a slip. Some people have better memories of the series than others, and Seamus and Dean are best friends and often mentioned together. Its a reasonable mistake to make for someone who remembers both of them, and they're not super difficult characters to remember. They're Harry's roommates, for crying out loud
"Oh but what if he only remembered it Dean because he saw the fake in scum chat" I don't need to read a fake claim to remember Dean Thomas. Nor to remember Ernie Macmillan or Lee Jordan or Katie Bell or Padme Patil or Parvati Patil or Blaise Zabini or Daphne Greengrass or Millicent Bullstrode or Anthony Goldstein or Terry Boot or Justin Finch-Fletchley or Susan Bones. Its just not a convincing argument