I dug up the book to check, and in the 20th anniversary preface page 16 and 17, a few lines before the previous quote there is :
"Mr Tortoise, meet Madame Tortue
A few years later, a wholly unexpected chance came along to make amends, at least in part, for my sexist sin."
And here comes the French translators part:"[...] they rather gingerly asked me if I would ever consider letting them switch the Tortoise's sex to female."
So I don't see what's the contradiction you're trying to point out.
Then it's probably my mix-up. An extremely similar passage occurs in Metamagical Themas, in which Hofstadter describes a conversation in which he first realized that he may have only assumed that Carroll's Tortoise was male. I've likely confused one with the other.
"Mr Tortoise, meet Madame Tortue A few years later, a wholly unexpected chance came along to make amends, at least in part, for my sexist sin."
And here comes the French translators part:"[...] they rather gingerly asked me if I would ever consider letting them switch the Tortoise's sex to female."
So I don't see what's the contradiction you're trying to point out.