Take a look at the end of your record. What is outeniqua.co.za doing there? You might have configured your DNS entry incorrectly to include your domain. I don't think the checker is going to handle that necessarily, whatever it means.
The page still throws the error for us, or rather it would if I hadn't dismissed it long ago, which I must have done. When clicking on it now, it still thinks -s is wrong, even though both articles I linked in my first post now confirm that it's correct (back in 2019, one of the articles was in disagreement).
The checker just isn't that great in general. When Skype for Business (online) went away last year, three of its four DNS records became unnecessary (only one had relevance to Teams), so I deleted them. I now have three red marks on that page for having done that.