I seriously doubt there's any convincing scientifc study, or even half-way decent statistical analysis, to support any (or even all) of the numbers.
However, I think the general folks have rightly observed that during most real fights, someone gets off balance and people fall down, or are pushed down, or are pulled down, or are thrown down ...
Is the percentage overwhelming? If you count every-time that someone is in a fight and during the fight someone is supported on the ground with body parts other than feet, I see no reason to disagree with it.
The question remains "so what?" That is, how does this "fact" matter to our fighting arts?
One answer, the one usually offered by the folks with ground fighting techniques to sell you for a low monthly fee, is that you must therefore have good ground fighting skills to be an all around fighter.
Another answer, the one usually offered by the folks whose art doesn't have much in the way of ground techniques, is that if you have techniques for clearing distanceand getting back up that's all you really need.
Even if we grant that 99.99% of all fights go to ground at some point, I'm not at all sure that we have near enough information to say that that fact alone should have any bearing on our training choices beyond the mere point that we need some way of dealing with that eventuality -- and that doesn't necessarily mean ground fighting techniques per se.