Yes, the slower flash synch speeds of older cameras require that you use broad depth of field and/or slower films and/or that you overexpose. (If using negative film, a little overexposure will not destroy your pictures as it does with positive film.) They also require that you use a steady hand to minimize the effects of camera shake at '60. Also, you are limited to lenses that you can hand hold at '60. The general rule of thumb about hand holdable shutter speeds states that this would be a 50 mm lens or shorter. I agree that the AE-1, or any older FP shutter camera, is not ideal for fill flash, but it can be done.
You are correct that all of the natural elements do provide reflected light. If this was not the case, any subject would be pitch black unless it was directly lit. Yet we see things that are not directly lit quite easily with a camera (and even better with our eyeballs). All light but direct light must be reflected light. However, I would personally not call natural reflected light "fill" light. The term "fill" implies to me that the reflected light was intentionally introduced for the photograph, by means of a reflector or a self-contained light source such as a speedlight. So, you are always getting light bounced in from the surroundings, but if you are needing to overexpose the environment, then it is not enough fill to give you a 1:1 ratio of the people to the background. The fill flash lets you do this.
If you try this out and like it, but do not like working around the synch. limitations of the AE-1, try the T-90, which has a '250 max. synch. speed. They are also much more rugged and full featured than the AE-1's, and they are not expensive.