Is this really in 3nf?
The example is provided in the answer in the linked question. The author of that answer - and example - states that (Pizza, Topping Type) is a candidate key of the relation - ther's also discussion in the comments below the answer.

So the example has the added restriction that every pizza has not more than one toppings of any type.

This may not be your usual definition of a pizza but with this assumption - even if we assume the common sense dependency that Topping -&gt; Topping Type - the relation is indeed in (2NF and) 3NF.