Huh?
The external SSD you buy on Series X can play every Series X game in existence and the future. You're not making much sense, the real difference is that the PS5 drive is of better value because it's faster, however the word "VALUE" changes when both are executing the same results for their respective machines. Series X games running exactly like Series X games would on the internal drive, and PS5 games running exactly the same as they would on the internal drive, so it's just paper value since from the gamer's perspective, they're both achieving the exact same thing.
On a side note, speaking of speed and value, let's take the recently analyzed game: The Ascent. On Xbox One S and Xbox One X, it takes 2 minutes and 47 seconds to load (Holy crap) that's almost 3 minutes of your life, meanwhile on Series S and X, it takes just 27 seconds to load, that is a major major difference and the major part of the benefit of having an SSD, and as we've seen in loading comparisons, the PS5 and Series X are not that different when it comes to loading, both offer an incredible upgrade from last gen and THAT is what gamers are seeking when they add these external drives.