I don't really understand that problem, exactly. I'm not aware of any restrictions for using AWS Identity Center (SSO) with an email address that happens to be a root email for another AWS account.
I checked the documentation but I couldn't find anything to show this to be a problem other than that the practice is discouraged.
Some tax forms like NY State fill-in forms have some scripted functionality that requires Adobe Acrobat to work, and they block rendering from other PDF programs. Even worse, once you've filled in the form, their PDF then blocks Acrobat from exporting to a normal PDF (the way you can save a print preview to a PDF for later) and so the only way you can get a real PDF of it is to print it directly and then scan it back to your computer.
Aerc lets you do this, although it doesn't automatically fill it in based on the recipient in a reply. So you have to remember to adjust the sender for each email that you send.
I think the parent was correct in the use of the word "Risk"; it's different than your definition, which appears to be closer to "likelihood".
Risk is a combination of likelihood and impact. If "risk" were just equivalent to "likelihood" then leaving without an umbrella on a cloudy day would be a "high-risk situation".
A rational person needs to weigh both the likelihood and impact of a threat in order to properly evaluate its risk. In many cases, the impact is high enough that even a low likelihood needs to be addressed.
I checked the documentation but I couldn't find anything to show this to be a problem other than that the practice is discouraged.