To what degree is cardio/endurance training sports specific? Are there any academic references detailing it?

**Background**:

I recently (a little over a month ago) starting training in Brazilian Jiu Jitsu (BJJ). When I was trying to looking something else up, I came over [this](https://martialarts.stackexchange.com/questions/2021/how-to-build-up-endurance-when-rolling) question here and more than one answer suggested that cardio is highly sports specific. But that seems to contradict this [article](http://www.grapplearts.com/Blog/2012/04/cardio-for-the-martial-arts/). It is also at odds with my own ***very limited*** experience since my running improved noticeably after I started BJJ.

So, I'm certain there is some degree of cardio being sports specific, but to what degree is it true? Are there academic citations? And if it is highly sports specific, is there anything that would help for those of us that can't make it to the gym as often as we like due to schedule conflicts (single person drills, etc)?

Thank you.