Really Rare Coin Of Caracalla?

I don't think those citations should be interpreted as necessarily representing all the examples extant. And in any case, generally speaking scarcity alone is never a guarantee of high value. That said, I think the coin being offered is, with that eagle motif reverse, a quite attractive one and graded very conservatively.

With some issues, there are countless minor varieties but they total to many thousands of extant coins of the type. These eagle tetradrachms were used as currency in a large city and were issued in large numbers. Unless it is an unlisted variety, it won't command very high prices. With 8 cited examples, you can be quite sure more are out there as well so it isn't really that rare. Think about the types that were only ever issued from a single die in a provincial mint...for them, the rarity commands a much higher price.