"Those who know Git mechanics" in this case is talking about extremely simple Git mechanics. Those who know more advanced Git mechanics would know that even a rebase is not sufficient to solve the problem of having pushed up secrets.
Aside from the obvious problem of all the forks and previously-cloned copies, the offending commits will still also be available on GitHub (at least until the next garbage collection), they'll just have the message "This commit does not belong to any branch on this repository, and may belong to a fork outside of the repository."
Any links that include the old hash will still be available online and will still turn up the code you tried to delete.