In my Logic class, we had a chapter on formal Aristotelian Logic, with its “Square of Opposition” (analysis of the forms “All X are Y”, “No X is Y”, “Some X are Y”, “Some X are not Y”).
The phrasing “All X are not Y”, we were told, is a traditional phrasing that is an artifact of its literal translation from the original Greek (or Latin?) wording, where the phrase is not ambiguous as it is in English.