I think the parent is referring to how you write PHP templates, which are HTML at the top level, but escape to raw PHP code inside particular delimiters (<?php>), whereas for JSX you write JS at the top level and 'escape' (kinda) to HTML in your returned element. Not exactly an inversion, but I can see the connection.

Really just the weird mix of xml and code

PHP is sticking code (php) in html

JSX is sticking xml (html-ish) in code (JavaScript)