ECL probably doesn't matter unless you are aiming for 19-20, once you've found the level you want to stop at... you just stop and roleplay... but that digression is a different argument. Probably best not to go into that on my part.
I assume it's going off the dragon magazine #319 statistics, in which case it does say it's meant to be ECL+1.
As for comparisons between other ECL's, well, +2 STR and +4 CON is a fair bit compared to an Aasimar at +2 wisdom +2 charisma and tiefling at +2 dex/int -2 charisma... so there could be justification there in rebalancing it. The ECL for a Dark Sun character could be considered "wrong" for what this server is aspiring to, Dark Sun has some whack things and it's not like Dark Sun material was written with Ravenloft or Neverwinter Nights in mind.
The fast healing they get is uniquely potent, the thing that makes them them, like spell resistance and darkness casting on drow. I don't think the additional ECL is penalizing them for that, particularly, since it is only unlocked at a high level and by then, you should have casters in your party already providing you with extended and far more potent duration regeneration, and Combat Vigor is just dumping fighters with flat +5 regen as long as someone slaps them in parry mode...
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Yeah I'd say it's just +2 ECL because of it being a +6 attribute score race. Drow is a +4 total race with spell resistance tacked on, ECL +2. Tanarukk has... All that. Attribute scores that equate to 0, but 6 points of which can be mostly ignored for physical combat, along with spell resistance and improved AC.
It probably warrants ECL +2, but compared to some other ECL +2 races it won't be the "best" ECL +2 race.
I'd honestly prefer it if it had remained at ECL +1 so people can explore Mul for what it is without feeling like the character that is meant to be the most hard working and physically sturdy of Athasian races that only requires HALF AS MUCH REST isn't.. ending up being the weakest of someones' group over time.
If any race is meant to never feel like they've reached "Blind Drive" it has to be this race. It's part of their staggering endurance that they would remain ECL +1 over ECL +2, a quality of the subrace itself.