Yeah it's an interesting one. By the letter of the law it is a red cos Virg (just) makes contact with the player before the ball. But in the spirit of the game (or common sense if you prefer) it's not, cos he is playing the ball and it's actually a ball hair from being a brilliant tackle, and to get sent off for such a thing is incredibly harsh.
In the past, when refs had more leeway to use their common sense, you'd almost never get sent off for an honest tackle like that. Even today most refs would settle for a yellow, if that, even though the law strictly means it's a red.
I think it's a great example of how good refereeing actually involves a bit of flexibility for refs to use their judgement and feel for the game, and not just be robots slavishly enacting rules without any judgement or common sense. No-one really even argues for this anymore though. In the name of "consistency", there's a general feeling that subjective judgement needs to be (as if it can!) taken out of decision. So you get decisions like this, which are technically correct but just not in the spirit of the game.
It's one of the reasons why the art of tackling is dying out. Everybody's afraid to make a last ditch tackle cos they'll get sent off if it's out by a ball hair (plus of course we've now accepted that it's ok for a player to dive if there is any contact whatsoever, although obviously Isak didn't dive in this case).
A tackle like that should be a foul, yes, cos he touched the player first. So it's a free kick or, if in the box, a pen. But the ref should be able to use their judgement and see that the player genuinely played the ball and was out by about 1/1000th of a second, so no red, yellow at absolute most.
That's what it should be, but we are no longer giving refs that leeway - rules say it's a red, refs aren't allowed to make allowances for intent or how close it is to not even being a foul at all.
Hence why the pundits are nearly all agreeing it's a red, and why the ones that don't think it is struggle to say why. (Carra's argument for why it wasn't didn't make any sense, for example. Andros Townsend knew it was ridiculously harsh but couldn't say why it was wrong, so sat on the fence.)