Is Earthshaker+Magnus imba/op ?

https://www.youtube.com/watch?v=MzV_Tb01Gy4

Slardar: lvl 13
Drow Ranger: lvl 17
Phantom Assassin: lvl 15

is basically erased by

Magnus lvl 12
Earthshaker lvl 11

within a few seconds.

How? Why? Just Imba ???

All AOE spells + reverse polarity (or vaccum for that matter) is imba.
The deadliest magnus combo is RP + ember spirit with empower and battle fury(s) that basicly wipes anyone caught within sleight of fist.

Big teamfight spells own midgame if you land them properly, especially in combination. Against competent opposition this is not easy - good players don't stand grouped up when there's heroes like that on the other team.

The other thing to remember is that these combos become less powerful in the late game - the damage part doesn't scale, so tanks and carries with plenty of equipment and usually survive them pretty easily.

First question that rises here is: how the hell did Drow and PA get stuck in one RP without popping BKB? o.O
Obviously its a good combo if they can't pop BKB before RP, but there are better combos with Magnus.

magnus(mid)+sand king(support)+ember(safelane carry).

you can hear them cry

Well right from the start, Slardar and PA were easy targets. All ES or Magnus had to do is make sure they're either clumped up (which they were) or somewhat close to each other and either of them (ES or Magnus) can blink in and ult (which ES did). Earthshaker had a Blink Dagger and took the window of opportunity to dunk them before they can react since they were so close to each other. Since ES can stun after casting spells at close range with Aftershock, the clumped up carries literally cannot do anything and got blown up. None of them had BKB and even if they did, they couldn't have activate it fast enough before they get blow up by ES since it is as you've stated on that video a low skill game.

sry, what does RP stand for?

Reverse polarity.
It is pretty annoying to pronounce and write all the time, so it is commonly referred to as 'RP'.

RP stands for Reverse Polarity. Also, doesn't the stun and grouping of RP go through Black King Bar?

It's not OP. It's great if you can land it so it's arguably OP in PUBS, but in coordinated play it's not that good. You put a lot on mid game execution, so if your mid game is bad it's not that easy to come back. Also easy to counter with silences.

Also, it makes the laning really weak for the magnus.

But hey, it's fun, destroys pubs, isn't Riki or Pudge related, and having a lot of disables IS in fact powerful, so why not?

It goes through magic immunity so BKB doeent help much.
The initial echo slam damage also goed though BKB, the echo damage wont though.