Perhaps you are thinking of Facebook?
Do you have any insight onto how much of the templating aspect of PHP is used at Youtube? I know of some large PHP installations where PHP is used as a general programming language that does produce web pages, but the templating aspect of PHP is pretty much minimal, and other engines actually produce the final output.
My point is that PHP pretends to be a templating language, then falls flat on its face, so people end up implementing much worse (Smarty, MediaWiki) or sometimes even better (Twig) templating languages on top of it.
In other words, it's simply not true that PHP's "declarative "templating" is the easiest, simplest, most direct way to generate html", or there wouldn't be a need for Smarty, the MediaWiki abomination (which is 17 years old but still widely used on 29 million pages), or Twig.
I'd say "to hack together something half-assed that barely works, but does provide the feature" perfectly describes the PHP development process itself.
I've always liked that youtube uses PHP - much derided, but its declarative "templating" is the easiest, simplest, most direct way to generate html. Maybe that was a factor too.
But I think the youtube engineers might have a more accurate opinion on why they were quicker, than the google video engineers.