What's a directional coupler?
What do they look like?
Don't bother answering those questions.

Why do the arguers, when caught in a tight corner, always escape to UHF for
help from directional couplers?

There are NO directional couplers at HF. They are as scarce as real swr
meters. So they cannot be used in futile attempts to explain what really
happens at HF.

You're next move will be to drag in scattering-matrices.
---
Reg ;o)