Quick Latin lesson: Reductio ad absurdum. That's what you've been reading. Ryan's argument is that if you can make a small segment's potential contribution to the whole seem small, then you shouldn't press for more from that segment, regardless of how little it gives now. If that were legit, then the Baynton-Bachmann argument for nothing from Baynton would be legit, too. Preposterous.
But wait. What about the idea that enough money to run the government for four months isn't very much? Four months is one-third of the year, and Ryan's hypothetical case raises it from about 1/400 of personal tax returns. So his example actually shows that even if you taxed the top 1/400 at a lower, reasonable rate and then asked profitable corporations to bear a significant proportion of the whole, the contribution of those two segments would be substantial. The tax burden on the rest of us, even assuming present spending levels and a balanced budget, would be far lower than what happens with those two segments making small contributions.
If you still think Ryan believes millionaires lack the potential to contribute in a big way, ask yourself this: When it's time to finance his next campaign, will he bother with the millionaires?