I think this is a bit of an overstatement. Calypso is a rewritten "dashboard" view and administration panel -- it's a client-side single page web app in React,

I think this is a bit of an overstatement. Calypso is a rewritten "dashboard" view and administration panel -- it's a client-side single page web app in React, with "a thin layer of Node.js on the server to build the initial web page," as they put it. This is communicating to the actual WordPress blog/CMS engine over a REST API, that engine is still in PHP, and I suspect it's going to remain PHP for the foreseeable future.