Amphetamine is a molecular structure classification, beta agonist is a classification based on the effect the drug has. Clen is a beta agonist that isn't an ampetamine, ephedrine is an amphetamine that's also a beta agonist. It's like all squares are rectangles, not all rectangles are squares. All aphetamines are beta agonists (or at least 99%).
So, both you and BBB are correct. MDMA is also in the amphetamine class and is a beta agonist (and psychadellic). Ritalin is a beta agonist that isn't an amphetamine.
Amphetamine really refers to one drug. Technically the name of the molecular classification is "subsitituted amphetamine." I think that's where the confusion comes from.