Lenovo's devices division is ensuring tha phones from the Lenovo's sub brands are all receiving the Marshmallow update. The Lenovo's ZUK Z1 phones are receiving the Mashmallow updates. A look up on Weibo shows that the ZUK Z1 phones are running the Android OS.
The Lenovo ZUK Z1 comes in two versions - A Chinese version and an international version. The Chinese version of the phone runs on the ZUK's own UI, the ZUI. The international version of the phone on the will receive a Cyanogen update.
Users of the international version of the ZUK Z1 will see the Android 6 on the Z1 for Chinese users. The ROM on the Z1 will get the ROM is a ZUI version. It is time till the Cyanogen OS updates will come to the Android 6 on the international phones.
The ZUK Z1 launched earlier this year with official CM support, Snapdragon 801 chipset, 3GB RAM, 64GB internal memory, fingerprint scanner and 4000mAh battery. Despite having looks not everyone will appreciate, the phone has found lots of takers owing to great performance and battery life.