The word "base" here is referring to the skills and attributes that one develops in participating in a sport.
You are comparing guys who are naturally athletic and successful participating in Rugby vs guys who are not naturally athletic and relatively unsuccessful competing in MMA, which is a false comparison. If we are going to compare Rugby vs wrestling/judo/BJJ/etc, then you must start with an equal platform.
If you take the same unathletic guy and have him train in a TMA he will beat the brakes off of the version of himself in Rugby. Likewise if you start with a natural athlete and have him in each. Sure, in rugby they will probably be better conditioned, but in a TMA they will be FAR better skilled for a fight.
Too many people in here seem completely ignorant to the meaning of the word "base" in this context. Without a doubt a natural athlete will have a huge advantage regardless of what he trains, and without a doubt a guy who trains in any endurance based sport will have a huge advantage over a couch potato. That does not make it a good "base".
"Base" is suggesting whether the same person will likely perform better in MMA after training in one sport vs another, and you have completely lost your mind if you think the same guy would fight better after training rugby than if he trained any of the common MMA elements.