The missing component of your analysis is that amazon has 4th option: re-sell instances of B as instances of A when A is more expensive, and otherwise allowing the market to adjust. The analysis is strictly limited to instances where amazon could, in theory, do this (e.g., reselling c6gd as c6g).

Assuming the market is in equilibrium, the above scenarious aren't realistic, as demand at the market price would equal supply at the current price (roughly, of course).

Suppose there are 1000 c6g and 200 c6gd, with equilibrium price of $3 and $2, respectively (i.e., all instances have demand). Amazon re-SKUs c6gd as c6g until there are 1100 c6g selling fro $2.90 and 100 c6gd selling at $2.90. Total revenue is $3480 vs. $3400. Of course it's impossible to know the true numbers without hidden knowledge of the market, but this is more akin to what would occur. Amazon effectively has a risk-free arbitrage opportunity here, so it stands to reason that there is revenue to be made. Customers don't have this option (since you can't short spot instances), so the best you can do is diversify and save money.

Edit: Actually, the AWS spot market is often out of equilibrium in a way that makes this reselling even more effective. For instance, in the example in the article the c6gd instance is actually pegged at the minimum price, so some number of those instances could be resold as c6g without moving the c6gd price at all.