It's a continuation of the previous work on quill (#6937 (Inline style in RichTextField content doesn't persist in quill) .
There are two different kind of block blots (blots that contains actual content either text or otherwise) in quill, one is Block itself and the other is BlockEmbed. My work with containers and formats so far addressed Block blots but not BlockEmbed. With container feature activated, working with BlockEmbed may break quill, so I cannot really make a pull request in the upstream until I make it work with BlockEmbed as well. Next thing to look out for.