Why do you think GP is saying or doing that? I think you've put a lot of words into their mouth that aren't there, and then got yourself worked up about it, enough to accuse them of ignorance and small-mindedness (at best).

I see absolutely nothing in their comment that even weighs in favor of FP, let alone saying other modes of thinking can't coexist. For all we know GP is a rabid OOP fan but sees the value in immutability (which FP certainly does not have a monopoly on).

I hope you can understand why benchmarking OCaml against Java isn't necessarily fair, since they have two completely different implementations (the latter being possibly the most mature set of JIT compilers and runtimes ever built). A more interesting comparison are immutable DAs in languages like C++, for example the immer library which has some very impressive performance characteristics for the problems it solves. And other applications like immutable ropes used in text editors which have extremely high performance compared to mutable variations. At the same time it's not a silver bullet, plenty of other immutable DAs have issues that are non-negligible and applications domains where in-place mutable DSs are required like hard-realtime.

Software engineering is about tradeoffs and not dogma, and that's what I was getting at. A take like "immutable is slow" is the counter dogma to "immutable is perfect."