The thing is - they don't (figure it out, that is).
I haven't seen anybody, physically talented or not, figure this out in a conventional way. OTOH, there are a number of people around teaching who have figured it out the "regular" way.
There are all sorts of IS skills out there, some of which can be achieved through athleticism, but have drawbacks.
Here is a short and non-comphrehensive list of IS skills
getting under without physically dropping
unbalancing on physical contact (doesn't require a strike, windup etc)
lack of feedback on contact or when waza is performed
able to generate power from very weak positions without the benefit of structural alignment