From what I remember, there were two "Enter/Return" keys on IBM 3270 terminals. One was the regular "Return" key we have today, which just advanced to the next field, and didn't submit the form. There was also another "Enter" key where the Right Ctrl key is today, and that submitted the form. So, I presume, instead of being against Tab key, IBM might be against "Return" to be the form submit key as people who use 3270 would expect it to advance to the next field.

And that was true for many DOS programs. Pressing Return would just go to the next field, not submit the form. That was one of the things that needed some getting used to on Windows.

Your memory is correct, and it's interesting to note that on the IBM terminal keyboards, the Enter key was marked "Enter", and the return/new line key was marked "↵". On the classic IBM PC keyboards such as the Model M, the Enter key is marked "↵ Enter". I believe IBM chose this to convey that the Enter key on the PC was both an "Enter" _and_ "Return" key in one. As you say though - individual applications got to chose what that meant in practice, leading to inconsistent behavior.