I've actually had a successful test with the water method. The lead coloring shows immediately and remains dissolved or in suspension or whatever for days, possibly forever. Turmeric doesn't usually color water at all.
I just tried this test with some turmeric powder I had at home and it failed according to what is shown in the video. I happened to have a lead testing kit, and it doesn't have lead though. The powder does have some ginger and other herbs in it, so apparently harmless adulterants other than lead can cause it to fail this visual test.
But apparently it's also not very soluble in water, so the question about sinking stands.
Possibly, a chemical powder is much finer than a ground-up plant matter, and takes longer to settle.
https://en.wikipedia.org/wiki/Lead(II)_chromate#Safety_hazar...