You are correct as far as it goes Wayne. But in this instance there is no significant power being transferred and the outward deflection will work fine so long as the angle to either of the pulleys doesn't get too shallow. Granted squeezing the belt toward the center does provide for more degrees of contact around the pulleys, but perhaps not too many more depending on amount of deflection in the system. At the same time, the set-up I have sketched is not to be taken as truly proportional, but rather just representational. Less deflection of the belts than my sketch shows would be desirable. Also note that should the set-up I've depicted be used for a clockwise running system, the idler pulley would be outside the belt loop pushing down (inward) on the upper belt, in a configuration similar to what you show, only with the opposite direction of motion.
The fan belt on my old Toyota pickup is set up much like my sketch shows, with the generator acting as the adjustable belt tensioner (equivalent to an idler pulley), in approximately an equilateral triangle configuration, which is actually much more open than what is shown in my sketch.
Meanwhile, you are showing your idler on the tension side of the belt, where it will try to kick out harder, rather than on the slack side of the belt where it will be mechanically easier to add the needed tension, whichever way the belt is deflected.