After using booking.com for hundreds of reservations over the last 10 years, my latest booking with them I've been asked to verify via Superhog - who I have never heard of.
However having seen all the personal information they are asking for, and the total lack of guidance on their website for those booking, there is absolutely no way I am giving them such sensitive details. I've told the seller this, and if they cancel my booking then that is there loss - plenty of other options around.