If you are interacting with a modern log-in box, where there is a first state to input the user name or account ID and then a transition to a separate state presenting a password input for this account. You hit the back-button. What do you expect? The box switching back to the input of the account ID, or leaving that site altogether to wherever you came from (e.g., Google)? I guess, most would expect the former, because there is change of state and a transition. I think, adding another sibling, thus changing the form on our own account, is such a change of state, as well. (In other words, we may want to revert this step, but we do not necessarily expect CTRL/Command+Z to work in this context, like it does for a "normal" editing task. But we may expect the back-button to work. Not to the least, because we had to press an action button in order to arrive at the present state.)
Moreover, this is pretty much how forms have worked for the first 20 years of the Web. (I.e., the browser stores a snapshot of the state of the page just before any transition or navigation event and restores this on the press of the back-button. Before there were single-page approaches, this was well defined.)
I don’t see those two ideas as equivalent, and so I don’t agree at all.
I can imagine a user seeing a page to input their email address and a subsequent page to input their password as two different pages, and so navigating back from the password input page should take you to the email input page.
I don’t think a user would expect to return to a page which models n-1 of a kind of a input when navigating back from a page which models n of a kind of input because conceptually they are the same thing.
They are not to the least conceptually the same thing: this is the form you customized by interacting with an action button. You expect this to change the state, and you do this either for a purpose, or by accident (in which case you may want to revert this, as you may fear that you've just broken the process.)
Moreover, this is pretty much how forms have worked for the first 20 years of the Web. (I.e., the browser stores a snapshot of the state of the page just before any transition or navigation event and restores this on the press of the back-button. Before there were single-page approaches, this was well defined.)