Shrewd business move for Hagerty to put out an article like this. It pre-emptively points Hagerty customers to the Hagerty Price Guide mechanism, in which value varies a lot based on condition.
The other comment I will add is that their analysis seems to compare two different buyers. I would think that people buying cars elsewhere and within a year selling on BAT probably did not buy the car with the intention to keep it. These buyers are not willing to pay a fair market price, because if they had to pay a market price there would be no room for a profit on a flip. Thus, this data set is skewed toward below market purchasers who resell within a short time. People who purchased on BAT and sold elsewhere within a year are probably not a comparable group because BAT is not a place bargain hunters search for cars to flip.