Migration paths are created on an as-needed basis. For sites without migration paths, moderators can always migrate anywhere, and one can always flag as "Other" and explain in the free text box that it belongs on whatever site. When we notice a high number of migrations going from one site to another, we activate a user-selectable path.
It turns out that questions posted on Magento SE very rarely go to SO; there have been like two migrations in the past year, so there's no need to enable the Magento → SO path.
In this specific case, I also disagree that the post should be migrated at all. The question states what the desired behavior is but doesn't lay out what the asker's existing attempt to solve it looks like. The general philosophy on the network is that only questions of high quality should be migrated, regardless of on-topic-ness; low quality questions should be edited first, or closed without migration if they can't or won't be edited.