The idea is that the callin player can play aggressively enough that you in fact DONT have a counter out by 13 cp. Assuming each player has accumulated around 3000 manpower by 13 cps, the callin player can use 2300 manpower to apply pressure to the opponent then have the heavy tank. The teching player has to fend off this 2300 manpower army with, presumablu, a 2300 manpower army of their own. This leaves them 700 manpower to tech and get sufficient counters to the heavy (which is often not enough). Then you have to consider the fact that the callin player can usually get more light vehicles (like t70s) which the opponent has to answer, putting on more pressure and giving the teching player even less room to work with by the time the hwavy tank comes out.
I'm well aware of that, but if the call-in player doesn't invest in tech and gets more armor, he'll be at major disadvantage after that initial heavy use and for that reason I don't consider it as a problem.
If you're in a state, where call-in at 13cp seals the game for you, you likely were losing for quite some time now to not outlast it by 3-5 minutes, which would be the time for that tech fuel to start paying for itself with multiple armored units.