I would guess that they say not to use it on a CPU as it probably doesn't have as good thermal properties as their other products. But remember, they are referring to 3GHz Pentium4s and AthlonXPs now, they aren't expecting people to want to glue a heatsink to such an old old old old slow slow slow CPU like your 68030.
From the review I saw, I don't think that thermal conductance is the big problem. Considering how much better it did than thermal tape I would imagine that it would do better than the little thermal conductive pad you find on many retail heatsinks. I think the big problem may be weight. Modern CPU coolers have quite a deal more heft than the kind of heatsinks this adhesive is designed for. The adhesive might not be strong enough and even if it is, I doubt it's a good idea for the CPU itself to bear the load of the cooler
