This sounds like lysenkoism.
Humans aren't the endpoint of evolution. Something will come after us, and if we're spread out on a ton of planets, there would need to be explicit counteracting forces (genetic modification, tremendous volumes of interstellar human travel, etc.) to make sure whatever comes after us is uniform among our interstellar backyard.
When it comes to neutral mutations, we can literally see constant variance creation in plenty of non-coding areas of DNA over time.
Drift occurs at a fairly consistent rate that reflects the intrinsic error rate of the particular replication machinery that a given organism uses. You can measure the statistical error rate of different ribosomal complexes.
Different planets are going to have different selection pressures. They'll have different conflicts. Different crises. Different cultures. Different reproductive preferences. Imagining that these populations will converge on the same wildtype by sheer chance is lunacy.