People hate(d) PHP for the developer experience, not because it can't or shouldn't run massive websites at scale. Anyway if NASA is using WP like most people do, the vast majority of pageviews are cached, meaning WP itself is more of a page builder than a server.

As someone who ran a digital agency for a while and inherited a lot of WP sites, I will say that there are few things more miserable in the dev world than managing a WP site with lots of custom functionality. It's the worst of all worlds (PHP without structure or version control, Turing-complete plugin architecture that makes security impossible, code running anywhere can monkey-patch any other functionality in the entire site, etc.)