class abilities that work like, but aren't, feats

[dragonhunterq]

Does having a class (or race) ability that grants you proficiency with a weapon or armour type work the same way as the feat of the same name?

More specifically, if a class ability grants you proficiency with heavy armour, but not with medium or light armour (unusual, but there is at least one way to do this), can you use heavy armour as if proficient? or do you need light and medium armour proficiency before you can use the heavy armour proficiently?

This faq makes it clear that they aren't actually feats.

[Andrew L Klein]

If you're proficient with heavy armor, anything else is irrelevant. You can use it proficiently because you have proficiency. There's no "as if" proficient, you are proficient.

Out of curiosity though, how would you get that without medium or light?