First time I've heard that. I can easily make my TCS kick in in a straight line. If rears spin quicker than fronts it will kick in.
This is the same on mine from my experience chris, the TCS will not allow wheel spin even in a straight line, it cuts in.
Definitely noticed that the ESP gives a little more tolerance in this weather, spun the wheels a few times no light comes on.
How odd, maybe there are different system revisions in different cars that allow for different slippage? Mine def kicks in very early, sometimes before I even think it might be about to slip. Was worse with my Rays than Rotas, but thats to be expected as the Rotas are wider so more grip. Maybe its a UK car vs JDM? Or even the year?