Yes. it is the crossing with the ruderalis that makes them autos. And, yes, since a part of the plant is basically ditch weed, the THC is generally less. I personally have never found an auto that I though was worth much.
This should tell you something. I do not know one single experienced grower that grows autos. In addition, those that I know that started out with autos (and some of those defended their choices vehemently) go to reg seeds after a few grows with autos and never look back.
I hate autos because we had "dumbed" down the genetics to (IMO) simply make it easier for lazy growers to grow. This at the expense of the quality of the genetics. Any time we take a step backwards and make anything "worse" worse for the sake of convenience, we should be ashamed of ourselves. We have gotten to be an "I want it right now" society with little patience to "do things right". It is a giant step backwards for cannabis.