Commit messages aren't mutually exclusive to the inline documentation.
I am making the case that inline documentation is far more important than commit messages.
Git was made with the intention that commit messages look like whole emails, describing not only why the change was made but also the thought process behind it, why this particular solution was chosen instead of some other etc.
What you describe is a decent header, although most people would probably prefer "Add ability to" (not "Added ability to", this is a custom that goes way back before git).
Inline documentation, or comments, is something else entirely. That is a moving target that can describe intended usage, remote APIs, and hard to understand passages. Commit messages describes a particular changeset, at a fixed point in time.
Google it, and see that most people's ideas of a good commit message goes way beyond the header ("shortlog") line.
If you use markdown Syntax in your commit body, github and other tools will gladly render it correctly. You can grep for commit messages with "git log --grep <pattern>" and it will also search commit bodies as well. It's beautiful!
When additional detail is helpful about the motivation for or gotchas related to a commit, starting a new paragraph and elaborating there is totally fine.
But I agree with you, they aren't mutually exclusive to inline documentation. The inline documentation is more important when the explanation is relevant to understanding the code which results from the commit, whereas a clear commit message is more important when the information is to illuminate the reasons for (or history surrounding) the change itself.
This also follows what I've seen at a few companies since then. Would you say that's a general rule?
There was only used by one person who gave no context on why he used that format, so it's probably no coincidence in such an example that to me the extra structure seemed to add only opacity and no real value.
I can absolutely imagine that being different in a company that used this convention widely and built tooling around it. Doubly so when a lot of the staff is junior enough that it's helpful guidance for structuring thoughts around what the commit does.
Though, I'm not sure it's a helpful restriction for commits that clarify, refactor, or clean up code. One can certainly reference an issue tracker number as the {feature/bug} element that gives the necessary context, but the strictness spreads the information around more widely than is natural.
If someone was adopting that convention as a workaround for helping team members learn how to communicate, they're probably going to find that it's a very incomplete workaround.
When merging you can pretty easily see what ticket each commit was for. Seems superfluous to put the same information in every commit messages.
By contrast, referencing the ticket number (but not necessarily ticket title) is common practice in the PR message, and seems much more helpful.
Commit messages are about the change, whereas comments are about the state of the code.