I'm just answering your question:

Since when does EasierDebugging = BetterProductivity?

Since forever. Because you're definitely need to do debugging.

He's not comparing programmer A & B. He's trying to improve himself via various factors, and screen size is one of them.

It's still not moot, because if as a company you have hired both Programmer A and Programmer B and give them dual monitors, you are still saving money (no matter how different their productivity).

Also - Programmer A will work for you only if you give them the best tools for the job - and that includes dual or large monitors. Otherwise you'll be stuck with Programmer B, and productivity hindered by debugging over a single monitor...

:-)