Java has memory safety and near native performance, the only thing going on for Rust is fearless concurrency which is a USP I think but not enough for me to bother learning its complicated syntax. Had I been unbelievably productive in that language, that would have sealed the deal completely. But alas.
That may not be important for what you're doing. But if it is, it's a huge pain in the ass to do on the JVM. For that reason, some people want to avoid Java altogether, not because it's slow, but because it has a lower performance ceiling.
But is that an argument against Java altogether? I don't think so. If your problem is solvable with sparse FFI, that is worth doing over throwing out a massive ecosystem you're familiar with. Value Types may yet come to the JVM someday, as ridiculous as that is to say now.
They said C# was fast and has tools for optimizing. They don't mention Java at all, which makes sense, cause Java is lacking in the kind of optimization tools that C# has. Even so, they come up with a list of reasons why you would still want to use Rust.
It's odd that you would only mention them asking the same question as the commenter, and not their direct answers to that question.
I left a sibling comment here on when you wouldn't use Java, I think that's a better place to start a conversation.