I wouldn't say so. Israel is thought to have nuclear weapons, but they don't admit it. There could be others. There is a huge incentive to publicly support a ban while secretly having the weapons.
Fantastic news! Been waiting for this since I've been with the CTBTO 13 years ago.. No testing on land of the pacific nations anymore! Hopefully the 5 big nuclear weapon states will ratify the CTBT in my lifetime as well...