Athletics teach you how to move smoothly and from your center. The modern study of athletic movement (as one would find in football, basket ball, track and field etc) teaches any of the things I can think of that are learned in "internal". The language is different but the lessons are the same. It's hard to go into anymore detail without further understanding your knowledge of sports training.
I think I mentioned this before, but Ark was a competitive gymnast before he started developing these skills and conditioning. And he would be the first to say that gymnastics had no overlap training wise, and in fact impeded his ability to learn his current skill set.
One problem lies in the language, you say "center" or "koshi" or whatever, we might say the same, but the physical meaning and development is completely different.
On one hand there's a specific physical development and control over the core, spinal erector muscles, obliques, diaphram etc that comes with the IS training, and then there's utilization of some kind of elastic property that develops in the body (whether its fascia or not is anyone's guess), not to mention a unique use of forces within the body etc, which, coupled together are simply "different" from what you see in modern sports.
Then you might say, "sure we use that too! Because I got my ass handed to me by someone that also uses those parts"
Well sure, we're only human with two arms and two legs, so the same parts are going to be used, but the manner in which they're used, and the way in which they are conditioned is going to be different.
I'd also caveat that "mechanically efficient" does not equal IS.
As in, I'm sure Vlad of Systema is extremely efficient at throwing someone using what someone perceives as being extremely effective or "effortless," but it still wouldn't be IS.
That doesn't make it bad or inferior, it's just simply not within the IS frame work.
Same applies to even some schools of Chinese arts, plus there's always shades of gray where some aspects might be used by some schools, but lacking in others.
I dunno, its just kinda obvious once you cross hands with someone that actually has it. Just because someone handed your ass to you easily, and got you to be able to replicate a couple of parlor tricks doesn't mean that the person had IS. If it were me, I'd go round a couple more "name" persons (not necessarily the ones mentioned here) and collect more data.
It's pretty cut and dry if you ask me, just go and check someone out already dude!