The amount of MMR you gain/lose depends on the average mmr of those you're matched against, rather than a static amount for a win/loss. If you lose a match you're expected to win (due to having higher mmr) you lose more mmr than if you lose a match you're expected to lose (due to having a lower mmr).
This means that if you happen to have a long queue time (resulting in potentially wider mmr gaps) you can gain slightly more mmr than average if you find yourself matched against relatively high mmr'd people (or gain slightly less against lower mmr'd people). This is how sometimes two people doing first time placements solo can occasionally have identical win/loss ratios but still differ by a division or two.
Similarly if they queue with people (as I strongly suspect they did), then the average of their premade is used for matchmaking, and so if they queued with low mmr people (as again I believe they did), they'd be matched against low mmr people, which hurts your mmr a lot more when they lose against them. This is likely why they managed to get placed roughly where they did despite their poor performance in placements. (Incidentally, a fresh account queueing with high MMR player[s] going 10-0 can theoretically get placed plat 5, although the premade restrictions make it fairly unlikely they'd be able to queue with sufficiently high mmr'd players for that to be the case.)
Also keep in mind your 'starting' mmr is roughly silver 4ish, so even solo 0-10 performances don't necessarily drop you all the way to b5, nor do 10-0 performances necessarily put you in gold.
