Does Bluetooth Mesh violate normal Advertising Rules?
After reading various documents I would have to say no.  

The key here is that the only valid "Relay Retransmit Interval Steps" are 1, 2 or 3.
So your first number is (((1-3) + 1) * 10) + rand(0-10) so your minimum is 20ms.

I would have found it just a little strange that a common "protocol" went against a standard it depended on.