That's a good point. You also need consistency in the chirality on asteroids (also with earth).
If we find one asteroid with chirality that doesn't match earth, it's good evidence that self replication just happens spontaneously rather than being seeded.
If we find only one asteroid with one chiral molicule that does match earth, that supports panspermia very weakly: it still might just be random chance. But multiple matching molecules, on multiple asteroids, starts to seem like evidence for a common source.
On the other hand: If we find multiple molecules with matching chirality, on multiple asteroids, and none of them match with earth... well, we should probably start preparing for the invasion.
If we find one asteroid with chirality that doesn't match earth, it's good evidence that self replication just happens spontaneously rather than being seeded.
If we find only one asteroid with one chiral molicule that does match earth, that supports panspermia very weakly: it still might just be random chance. But multiple matching molecules, on multiple asteroids, starts to seem like evidence for a common source.
On the other hand: If we find multiple molecules with matching chirality, on multiple asteroids, and none of them match with earth... well, we should probably start preparing for the invasion.