I'm sure there are a few dozen Dutch jokes based on the fact that bank and sofa are the same word.
Actually there aren't, I can't think of any. Probably because we have much better candidates, such as:
Als achter vliegen vliegen vliegen, vliegen vliegen vliegen achterna.
If flies fly behind flies, then flies are following flies.
If "sofa" is "bank", then why did I learn "zetel" & why is that wrong?
Zetel generally is mainly used in Belgium, plus it refers to a (comfy) chair and not to a sofa.
Well at least where I'm living in Belgium, it's a sofa.
As a rule of thumb, the course does not include words that are primarily used in Belgium. Other examples of that are appelsien for orange and nonkel for uncle.
generally speaking, sofa and couch are same things, right?
Yes. They are.