> not that you assume that the types of changes that can happen in a type of bump will happen

… an assumption that something happened is not a definitive statement that it did happen, only that we're assuming it did, because it could happen, or perhaps here, that because the major was bumped, that it is legal, according to the contract given, for it to have possibly happened in a way that we depended on. They're not saying that it will/must; "assume a major version is incompatible" is not at odds with what you've written.