I don't get why it would be difficult to understand but if I must: momentum = mass × velocity . p = m × v
Your third option is Bailly sitting off him which would allow Isak to run at him directly. That's exactly what you want to avoid as a CB even if you're fast because the attacker gets the advantage. I also completely believe he is right to go the man in that scenario, the idea of allowing a striker possession as a CB and knowing your partner is not that fast is a bit loco.
In an ideal world one of our CMs is a bit deeper or Bailly is a bit closer and makes the tackle or just makes soft foul to avert the danger but you're making out he's made some huge error when it's such a minor incident in the game and is actually proof of the benefits of having a fast CB. Recovery pace is king in football.