You're right, an FO couldn't run a command course or nurture a new Cpatain. I don't think anyone was ever suggesting that one could...
But a FO with more than a few years on the line, possibly having seen several sets of SOPs and operating environments might be equally, or possibly even more qualified to
able to show me stuff which is not necessarily covered in manuals, various 'gotcha's' that every plane has
than a Captain who has spent their entire career flying 20 routes to 6 countries in one airline/aircraft and got command/TRI sooner due seniority...?