Learn Spanish (Latin America): Esa afirmación no es más que un mito de la trivia. - That statement is nothing more than a trivia myth.

It acts as a restrictive negation. By framing the sentence with no (not) and más que (more than), it limits the subject strictly to what follows. It translates directly and naturally to 'nothing more than' or 'nothing but' in English.

Since the lesson covers both, could I use sino instead of más que here?

You could technically say no es sino un mito to mean exactly the same thing. However, no es más que is much more common in everyday spoken Latin American Spanish for saying 'it is nothing more than', while no es sino sounds a bit more literary or formal in this specific construction.

Afirmación looks like 'affirmation'. Is that the best translation here?

While it can mean 'affirmation', in contexts like this (discussing facts, myths, or arguments), afirmación is the standard Spanish word for a 'statement', 'claim', or 'assertion'.

Is trivia a standard Spanish word?

Yes, it is a loanword from English that is very common in Latin America. It is treated as a feminine singular noun (la trivia). You might also hear datos curiosos (fun facts) to express a similar idea, but trivia is widely understood to mean random knowledge or trivia games.