As you may know, the collapsible table of contents functionality is specific to articles viewed in the Messenger chat window. When viewing one in a messenger chat the TOC is at the top. For the regular desktop Help Center view, the table of contents appears to the right.
I understand that having collapsible sections in the article is an alternative, but it is a bit different. Plus, creating collapsible sections is pretty difficult/glitchy/not intuitive. Having the table of contents at the top right is good, not bad, but to my taste having it at the top center being collapsible is better. It would provide a cleaner, more focused experience similar to what you see in the Messenger view.