As much as I want to credit python, I doubt that was the cause. It's typically far easier to ship a new feature at a small company, because there's no bureaucracy, no wait for QA, no need to translate text into 20 languages, etc.
Additionally, people are more tolerant of mistakes made by small companies. Features stopped working? Maybe couple of people will complain.
Google's features stopped working? Media, bloggers.. everyone will have an opinion on why this happened and how to resolve it.
Anyway, I agree with your conclusion: big companies move really slowly in comparison with smaller ones.
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.
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.