That was confusing part of this video . May be there are some limitation on the tools he uses to tune

I don't think the number of the gear ratio really matters, what matters is that you know what it actually is (since every IK calc depends on said ratio); 8:1 is probably arbitrary and/or looks nice & might simplify some stuff.

	▲	eichin 3 days ago \| parent \| next [-]
		It might be a lot easier to check the ratio "by hand" (by counting rotations etc) if it's numerically simple. (IIRC in some earlier videos he noticed that the pulley size ratio wasn't producing the expected movement ratio, because they were built as an obvious 8:1 or 10:1 or something, and didn't match - which led to him figuring out the subtleties of the design - I can easily imagine wanting to preserve that aspect just for debugging, at that point, even if you now have correct math.)
	▲	mlhpdx 2 days ago \| parent \| prev [-]
		From a coding point of view it’s also nice if all the drives are exactly the same, so each isn’t compensated for separately. But yeah, just a nicety.