IS-IS is a routing protocol. It can forward on information about what direction to send traffic for a number of protocols, including IPv4 and IPv6.
The key point there is that it's information about what direction traffic should be forwarded but it's not the means by which the traffic is actually forwarded.
The simplest analogy is that of roads, road signs and addresses: the road sign tells you which road to take to get to a given address but isn't the road itself. The road exists whether or not there are signs and the simple existence of signs isn't an assurance that any possible type of address is reachable via a given set of roads.
In concrete terms the implicit item in your question is the distinction between the control plane and the forwarding plane. The control plane is concerned with exchanging reachability information between independent nodes. The forwarding plane is purely concerned with the act of receiving, analyzing and forwarding packets (or frames) on the wire. The control plane both drives the forwarding plane and utilizes it to actually exchange information but the two are ultimately distinct. A control plane protocol (...like IS-IS) can carry information about any number of subjects without the forwarding plane necessarily being able to handle corresponding traffic types.
So - back to your question - the IS-IS process might be carrying information about how to reach IPv4, IPv6, IPX, X.25 and CLNS addresses (and, yes, all of these actually have/had specs defined at some point) but this doesn't mean that the routers actually running the protocol are able to forward datagrams for all of them. IS-IS might tell you about your IPv6 routes but if some of the routers along the way can't actually forward v6 then it's not going to do you much good..