I believe such statements have the implied "to all known knowledge". Also, ow would you prove it definitively?

There's no reference to any systemic search for mirror life. You can't prove something doesn't exist, but you can actually search and document the methods used, who knows, it's probably all around us. It wouldn't surprise me to learn it's part of how fungi communicate or something like that.

It would be far better to find it, and document where and how it already works, wouldn't it? I know that goes against the funding paradigm we have, but that's shit anyway.