There's no hard line between one site and another. The borders aren't clearly defined, such as when crossing between one country and another; instead, some topics blur a bit and overlap.
Project Management SE is a site for questions in the field of project management. Managing project teams is one thing that is on topic. Thus, it's possible that a question asked here might also fit there and vice-versa.
The question you reference is also on-topic for Programmers SE since it's about requirements documentation. Now, project management is actually considered a field that stands on its own, and a competent project manager should be able to manage a software project, construction project, and other temporary endeavors without having domain knowledge or even getting involved in requirements elicitation. For instance, one key responsibility of a project manager is to manage the schedule. Scope creep is another project management topic. As such, this question is close to something that would be asked on one of those sites.
However, we don't migrate questions just because they might be on-topic somewhere else. If a question is on-topic here, then our first loyalty is to our own site. Instead, try to make the question fit our site even more via editing and suggestions in comments.
If a question is closed, we still would only migrate it if it's a shining example of a great Stack Exchange question that's so valuable that it would be a travesty to let it die. These questions are grammatically perfect, represent the best of Stack Exchange, and are something the target site would be happy to have on their site.
For a majority of closed questions, they'll likely remain closed. Posts that are off-topic tend to have other issues. For instance, this particular post is closed as "unclear". Hope this helps!