Darkness abilities count as magics, it's not a physical stat.
Not necessarily. We saw the commandments use the darkness thing to fly around and summon demons even when their power was virtually at 0. At least for the basic form of the darkness thing no magic is required. Perhaps it makes sense to consider the black mark as a limb which the demons can manipulate freely. And it wouldn't be weird that there are specific or more advanced darkness techniques which also require magic.
---------- Post added at 09:37 AM ---------- Previous post was at 09:28 AM ----------
Well we're not 100% he fought Zeldris 1 vs. 1, we just know he contributed of the Demon clan's sealing. I know he crushed Galan, but he wasn't expecting Meliodas to come back 100x stronger, by the time he got the 1st hit it was already too late for him since Meliodas clearly had the advantage. I'm sure if Galan was aware he wouldn't have been crushed this easily. I still doubt Meliodas was holding his power 3000 years ago because it was an enormous war. Maybe he used all his rage and the demon's were weaker enough to be sealed. I'm sure if they could have erased them they would have done it, so maybe they couldn't kill them.
Also Fraudrin survived Meliodas full power, so I'm quiet sure the best commandements can go toe-toe against Meliodas.
This is just assumptions. But I really think for the sake of the arc Nabaka is gonna show all of what Meliodas can do. Because if he doesn't that means there is no suspens in this arc, if the reader knows Meliodas can beat 10 commandements while holding back. I'm pretty sure Nabaka is gonna stick to the classic going all out + getting a power up in the next arc shonen technique
The big question here, IMO, would be whether the demons could heal and recover from injuries while inside the seal. If they were in some form of stasis then it would make sense that they would awaken pretty much exactly as they were before being sealed safe for their magic because the seal itself erased it. With that logic then we could make the case that the demons before being sealed were perhaps at least not significantly injured or anything from battle. However this is a weird scenario. Anyways, the only way this could happen is that the commandments were lured somewhere, a trap, and they feel for it and where sealed. Its plausible there was some fighting but not serious enough so as to allow them to pretty much be at top physical condition after being sealed.
The alternative is that the seal allowed them to recover and they were caught after being defeated. They would have healed within the seal. However that is improbably because the seal itself seems to have preserved them for 3000 years.
Meliodas with all his wrath could not even kill fraudrin. The commandments at their full power are all likely to be difficult enemies for meliodas and the sins. Even gallan should be more of a threat once his magic recovers. Unless his magic was already full though that sounds unlikely.