If you connect the phrasing of the several steps in the migration dialog, you get:
This question is offtopic because it belongs on x.SE.
These days, there are many sites with overlapping scopes so this reasoning is plain wrong; it only makes sense if scopes are disjoint.
In fact, there is no justification to put migration under "offtopic" at all; we can migrate perfectly ontopic questions (upon user request) because they may get more attention and/or better answers on another site it is ontopic on.
I would even argue that the current placement in the dialog hides migration; non-veteran community members may struggle to find the option when looking at an ontopic question.
Therefore, I propose to move migration to the top-most level of the closing dialog (or give it its own link/dialog) and communicate something like:
We migrate this question to x.SE because it may get better answers there.
We can always tell authors of truly offtopic questions so in a comment.