The score is a black box. There is no way any player can watch a person play the game and accurately guess the score short of having played a lot of GW matches and developed an intuition. But what we do know about the algorithm does not mean that “a fast team” is the only way to get a high amount of points.
It seems to be about some form of balance. Actually the word I saw used was “efficiency”. That means, to me, ideas like you should be casting your troops spells as soon as they fill and generally spending as much mana as you take in. That mana should also very quickly result in damage, they possibly compare mana spent to damage dealt. Calculations like that are done in MtG: generally 1 point of toughness per mana is an acceptable rate, and 1 point of damage per mana is likewise the par.
So a fast Doomskull team ticks a lot of boxes. It tries to spend all of its mana on a few setup turns, then deal lethal damage to 4 troops in as close to one turn as possible. It’s hard to win fast without being efficient, so yes, faster teams tend to score higher.
But “hard” doesn’t mean “impossible”. If you get points for spending your mana intake and dealing damage at a close ratio to that intake, a slightly slower team with high mana costs might do the trick. If the efficiency ratios I talked about above are used, doing 3 damage from 3 mana in one turn is the same thing as doing 27 damage off 27 mana over 9 turns.
We also know your opponent’s performance matters too. A lot of factors like skull damage dealt are compared between players. It’s easier to assume in a very short game the victor did the most skull damage by a wide margin. But a slower team, perhaps with excessive barrier, might end up achieving the same kind of ratio.
The truth is we just don’t know. The devs implemented it, it’s server side, and they intentionally designed it to obfuscate the mechanisms. I don’t think it’s as simple as “fast team vs. slow team”, I think the devs explicitly wanted more than one play style to be “winnable”. If there were a simple, every-color team like Doomskulls that did it, I’m pretty sure they’d rework it.
So if the person’s in your guild, ask them what they’re doing. They should share.
If they’re not in your guild, how do you know they’re playing a slow and less powerful team? A lot of my GW teams don’t actually use superstar mythics. He “shared” a team with you, but how can you be sure it’s actually a team he uses? Competitive people don’t share their teams easily, and I wouldn’t be surprised if someone I don’t trust showed me a bogus team just to shut me up. Even if he was part of your guild, it sounds like he didn’t share a lot, which again tends to indicate the kind of person who would troll his so-called friends.
So in the end you just can’t know. But I’d bet they aren’t showing you their real teams faster than I’d bet they’re cheating.