The note in the lesson says:

"This rule does NOT apply to sentences using the verb être and other Verbes d'état, with which the indefinite article doesn't change."

Note that the rule clearly specifies verbes d'état, of which rester is one. So the exception also applies to rester.

It's an example of emphasising the word "one".

Here is a quick explanation:

What are verbes d'état? S'il vous plait | French Q & A | Kwiziq French

And here is a list of the most common ones:

List of French "state verbs" - "verbes d'état" French grammar from Kwiziq French