Could be so, but maybe not. During my recent back problems (resulting in a fusion), my most comfortable position was on a road bike. As TSL and others mention, the upright position puts all the weight on one spot - your vertebrae lined up like ducks. A "laid out" position puts the weight in other places, actually reducing the pressure on the back, and even stretching the back muscles. However, this varies markedly with individuals, and may not apply at all to you. The only way to find out is to give it a try, either way.

Instead of a "comfort" bike, consider a low-end entry level mtn bike. Great flexibility in use, great gearing for a beginner, and you will keep it forever, even as you add more bikes. For example, Specialized Hardrock or equivalent.

Good luck and have fun.