This is false.
Perhaps in part due to my unorthodox builds(my Retribution character is a utility rogue), but I can always reliably have the AI target precisely the party members I either want it to strike, or can afford it to strike(there's only one of them that has no business attracting attention).
At the absolute worst, I can usually at least predict who's going to take the hit, and it's easy enough to ensure that it'll be the tank.
On what difficulty are you playing ? It may changes things.
I play on classic and I can assure you that the target system looks something like :
AI Prio 1°) Is it possible to AE several characters (like by using a surface, a spell, a grenade or a special arrow)
AI Prio 2°) Is there an undead in the team ? If there is, use a healing spell on him.
AI Prio 3°) Which character has the less armor/hp then focus it.
You can try to use every tactic that you want like the clouds or else, the enemy will just jump/teleport/move toward your weakest player and attack him. ALWAYS ! Thus making the tanking absolutely useless. There are already several topics on the forum regarding this problem.
On a funny part, i play kitting games with AI using one of my mages while my 2H warrior utterly destroys the enemy ranks without being targeted once. Funniest part, they still continu to follow my mage even at the price of giving my warrior a free attack of opportunity.
There is no tanking in the game, but there is kitting !