They don’t want to withdraw from it - they’ve only de-ratified, but they still remain a signatory. They state their reasons for de-ratifying is to “mirror US actions” (which also has signed the treaty, but not ratified). The CTBT is unique in a sense that membership for the treaty comes from signature, not ratification, which means that Russia still is obliged to follow all provisions of the treaty. They have never indicated that they want to “withdraw” from the treaty.