But you just described Sybil Attack which can happen with or without multiple implementations of bitcoin. It doesn't even need a vulnerability or hashrate to happen.
It's easier to perform this type of attack when you can get other people to voluntarily be your sybil nodes. That's what multiple implementations do in this situation: other people are voluntarily being the sybil nodes.
I may be wrong about this but the way I see it, like everything else such as block size this is purely about what is less bad.
On one hand we have a network of nodes that all run one implementation that if that has a vulnerability which is exploited the whole network will be crippled and the damage will be big.
On the other hand we have new different implementation(s) that might have vulnerabilities and might introduce attack surfaces that
will not cripple the network and we have ways to fight these attacks to some extent. So any damage done won't be near as big.
What is the damage of this sybil attack? Some exchange and the traders losing money? That is not new.
What is the damage of a vulnerability like this being exploited? We would be forced to do a "roll back" and lose immutability of bitcoin.