The threshold time is based on still air and OEI TAS (but not to exceed Vmo. The speed and the distance are to be specified in the OM.
The whole thing is a statistical safety issue i.e. if you have an engine failure what is the probability of having another fail in the following 60, 120 or 180 minutes - set maximum time to acheive required probability of safety.
Having the fuel to divert / continue is a totally separate issue and at planning based on expected conditions i.e. you could find that due to the winds you can not operate at the full 180min ETOPs distance from an alternate and have to route closer or with the winds in the opposite direction you could find that at the max distance you need less fuel than normal thanks to a big tailwind.
So the threshold time is a statistical issue. Having the required fuel to divert from the ETP is a survival issue!!!