It's 100% not release powder, it's part of the compound that's secreted over the age of the tyre.
Ask yourself this:
Did you ever see powder on any of your tyres BITD? No, not one of a hundred tyres you, your mates, the kids at school owned. Then it can't be release powder!
If the tyres are riders any secretion is worn off through riding as its such a slow release, only tyres that have sat idle for years exposed will eventually secret so called release powder!!
Can you tell folk calling it release powder makes my fookin teeth itch?!!!
Anyway in answer to your question, it's proper stubborn to get off, some folk leave it on but from my experience to get a showroom shine use a tyre gel from Halfords.