Another interesting detail about this sentence: When you say "Az almák nagyon nehezek!", it means that the apples (each separate apple is very heavy) are very heavy. If you say "Az alma nagyon nehez", it will mean that either the apples are heavy or the (that) apple is heavy. So, in Hungarian like in all Turkic languages, one doesn't need to use plural form when referring to categories of things. A good example of it can be "lab", "kéz", etc. (referring to hands and legs, even though it is in plural).
Is there a typo in this sentence? Shouldn't the correct way to say it be "azok az almák..."?
No, it isn't a typo in this sentence. "Azok az almák...", would be, "Those apples".
Sometimes "az" will act as a definite article, but only because the noun after it begins with a vowel.
Kavics is right, but probably just mistyped one thing: az means "that", not those. It's singular. This sentence may help you remember to avoid confusion in the future: Az az. = That's it. (Literally)
Thanks, @kavics6! Now, it makes sense. "a/az" is a definite article, so "az" means both "the" and "that".