Short answer: You are confusing two different issues: heritability within popula

Short answer:
You are confusing two different issues: heritability within populations and directional selection across populations.

Once tempo traits are established as heritable (which you concede), environmental gradients cannot produce long-duration tempo divergence without producing allele frequency divergence—because tempo traits are polygenic, not plastic-switch traits.

Environmental mimicry produces local, reversible, non-stable variation.

The observed pattern—persistent tempo differentiation, predictive PGS variation, cross-regional directionality, and post-migration persistence—is the signature of a genetic cline under long-term life-history selection.

To deny this, you must assert that selection operated for centuries or millennia yet somehow did not change allele frequencies—an impossibility under any quantitative genetic framework.

So the hypothesis is not underdetermined.
The environmental-only model fails parsimony, fails decidability, and violates the known behavior of polygenic traits.

LONG ANSWER available on request.