Great question 🙂
Technically the metabox is still tied to post object. But if that block is not added, then correct there would not be a value.
It is possible to use block template to make certain blocks required.
That is covered in depth in the Gutenberg theme development course, but I have a quick tutorial on it here https://wp.zacgordon.com/2018/01/05/how-to-add-block-templates-to-your-wordpress-theme-or-plugin/
Hope I understood your question correctly and that helps!