OK, I admit it -- I do usually top off my account (and thereby make Platinum) by flying some low cost legs bought at a sale fare. Face it: 100,000 miles is a lot of travel on one airline in a year, so routine must-do travel for a lot of people won't get you to 100,000 miles. So, I suspect a lot of people do what I do to put the mileage over the Platinum top.
Therefore, if you average in the cost of must-do business travel with that of cheap tickets to top off the account, I suspect a case could be made that the AVERAGE amount spent per mile earned for Platinums is less. However, I have no doubt that the total annual cost for all tickets is still greater for Platinums than it is for Golds.
Airlines often look at things like yields (i.e., revenue generated per mile of travel), so quite probably this was the context in which the cocktail party talkers were speaking.