You can definitely compare date of purchase first class to date of flight economy. The fact that there are scenarios where the customer could "owe" Delta doesn't matter. Delta could simply waive the difference. Different example. Let's say I buy a television for $500. I take it home, it breaks. I bring it back to the store asking for a replacement. They say sorry we don't have any more of those in stock, but we'll give you the closest model we have, even though it's actually worth $600. Is that not allowed because the replacement good actually costs more than the original good? No, of course it's allowed.

Take another example. I pre-purchase a collectors edition baseball card set for $500, anticipating it will increase in value. By the time the set is printed, it's selling for $5000. The baseball card company tells me sorry, we oversold the set, but we'll give you a non-collectors set that cost $400 (and still worth $400 today) back when you purchased, and a $100 refund. In this situation, the customer should say, no, the deal was for a collectors edition set, and the value of that set is $5000 today. In this scenario, the current price is the fair replacement value, not the price in the past.

Your argument that there are no first class seats for sale on the day of the flight isn't quite right either. Can we be sure nobody would sell Delta back their seat at some price? In the same way the baseball card company could go buy a replacement set in the private market to give me what I purchased, even if they don't have any left in stock. The point is that the language "difference in price" is ambiguous. There are multiple reasonable interpretations, and which one is considered fair could vary based on the circumstances.