Well as been said time flows different in the warp a csm could be only 500 yrs old real space time, but cause of the warp 5,000 yrs may have passed since the csm turned traitor and went to the EoT, but we know that most csm have had far more combat experience that most space marines, and there will be those that have had 10,000 yrs worth of experience.
It is in the fluff that those few in the traitor legions will be 10,000 yrs old.
Even a 200 yr old chaos marine my have the experience of 10,000yrs of combat.
One example could be Kharn the betrayer, because of Khorne, i bet Kharn is going to be 10,000 yrs old, same with most of the SC in the chaos dex.
And so its nothing to do with chaos fanboyism, when its in fluff.
I wouldnt put it past some csm champions to be able to put down even Calgar, i would most certainly bet say a dark apostle could best most space marine heroes.