That's definitely right. I have a heater in my 200l aquarium and you can see the hot water coming off the heater and hanging around there. The instructions on the box say to direct the water pump or some water flow onto the heater to distribute the warm water evenly. Some fish just hang out at that end of the tank because it is warmer.
In your fermenter, that would mean the brew around the heater would be fermenting at several degrees higher than the temperature on the thermostat...not ideal.
Much better to warm the water around the fermenter as you and Linz suggest. One thought though, you are supposed to operate the heaters vertically. Don't know if this is a safety recommendation. It may just be because it generates its own water current that way and it's perfectly safe horizontally. To operate it vertically you would need a pretty deep basin. You can't have any of it sticking out of the water or they crack.