From what I see, this does not help with pinning the dependencies and it doesn’t verify the downloaded action has the same content as it used to have. In other words, this is a tiny patch on a big wound.
We use commit hashes to pin actions, have the version as a comment (e.g # v4) and renovate will keep both up to date in the PRs.
And there is a more or less recently added repository setting to require actions to be pinned to hashes.
How does this lock down transitive dependencies? Is it effective if the action you rely on doesn't pin its dependencies?
This is the way to do it.
Pin by hash.
Verify that the actions themselves aren't pulling in unpinned dependencies from Actions, NPM, or elsewhere.
Have a CI job or bot create PRs for new versions. Verify those PRs before merging.
If any particular action becomes a recurring chore or risk, consider if you should keep depending on it.
If you do these things, the "we need a package manager" is moot and most if not all of the concerns in that blog post don't affect you.