It says: 'We’ve built an esoteric application (even by Lisp standards)'
The application was considered by the author to be esoteric in 2015, not the programming language.
Quote: "Our Lisp services are conceptually a classical AI application that operates on huge piles of knowledge created by linguists and researchers. It’s mostly a CPU-bound program, and it is one of the biggest consumers of computing resources in our network."
The application is the compute core of a cloud-based grammar checker with lots of complex lingustic knowledge, which generates a lot of processing load.
And "(even by Lisp standards)" just means that there are other applications in the Natural Language Processing domain, which were written in Lisp, like software to do machine translation between language or text content checking systems for technical documents.
He wrote: 'Contrary to popular opinion, Lisp is an incredibly practical language for building production systems.'