What I don't understand is why would chirality and panspermia be so tightly linked.
The data right now still leaves every option on the table just because having any ratio of chiral molecules doesn't have to define how life evolves. It can't answer whether those molecules formed on Earth or hitched a ride on an asteroid, or life itself formed here or was brought here.
We can assume that in a soup with balanced proportions of each chirality, the left handed molecules created a self replicating mechanism (some definition of "life") first or faster than right handed molecules, either accidentally or because some yet undiscovered advantage. Whether this happened on Earth, or was brought to Earth by one or more of the millions of asteroids is hard to prove.
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.
What I don't understand is why would chirality and panspermia be so tightly linked.
The data right now still leaves every option on the table just because having any ratio of chiral molecules doesn't have to define how life evolves. It can't answer whether those molecules formed on Earth or hitched a ride on an asteroid, or life itself formed here or was brought here.
We can assume that in a soup with balanced proportions of each chirality, the left handed molecules created a self replicating mechanism (some definition of "life") first or faster than right handed molecules, either accidentally or because some yet undiscovered advantage. Whether this happened on Earth, or was brought to Earth by one or more of the millions of asteroids is hard to prove.