This is grease.
It's primary use is to put on the splines. ISIS is a tapered spline. This means to that to lock it in place it needs to be forced up a spline to the lock surface. The correct distance from the point where the crank is placed on the ISIS spline to the lock surface is 3mm (experience tells us that the tolerance on this figure is quite large, between 1.5 and 5mm seams to work). To allow the crank to ride up that taper properly the grease is required.
Using a threadlock on the spline will reduce the easy movement of the crank up the spline and hence reduces the locking force that holds the cranks in place. It should not be used on the splines.
Threadlock on the bolt threads can be used. When new bolts are supplied most come with threadlock pre-installed on the threads. When re-applying a bolt after it has been removed you should re-apply additional threadlock or grease.
In our workshop at Unicycle.com in the UK we use grease as experience has shown this to be the most effective. The grease reducing the friction on the bolt thread, so helps the force be applied - so locking the system in place.