Yes, I agree that if everyone in the world is either my friend or my enemy, then “anyone who isn’t my enemy is my friend” is equivalent to “anyone who isn’t my friend is my enemy.”
But there do, in fact, exist people who are neither my friend nor my enemy.
If “everyone who is not my friend is my enemy”, then there does not exist anyone who is neither my friend nor my enemy. You can therefore say that the statement is wrong, but the statements are equivalent without any extra assumptions.
ISTM that the two statements are equivalent denotationally (they both mean “each person is either my friend or my enemy”) but not connotationally (the first suggests that most people are my friends, the latter suggests that most people are my enemies).
Yes, I agree that if everyone in the world is either my friend or my enemy, then “anyone who isn’t my enemy is my friend” is equivalent to “anyone who isn’t my friend is my enemy.”
But there do, in fact, exist people who are neither my friend nor my enemy.
If “everyone who is not my friend is my enemy”, then there does not exist anyone who is neither my friend nor my enemy. You can therefore say that the statement is wrong, but the statements are equivalent without any extra assumptions.
ISTM that the two statements are equivalent denotationally (they both mean “each person is either my friend or my enemy”) but not connotationally (the first suggests that most people are my friends, the latter suggests that most people are my enemies).