Everyone has a theory just like everyone has an opinion. Some people think Trump won the 2020 election. Other people think life came from crystals. Whatever, man. No one has a clue how life emerged.
Not only that, but Earth is probably the only planet that has life of any kind.
There is simply no explanation for the existence of life at all.
Nobody has the faintest idea how life came into existence, and the question of the origin of life is, frankly, the only interesting question because it is a prerequisite for the existence of any other question.
Django is so boring, stable, and well-understood, and there are so many solid code examples, that ChatGPT is basically a super-powered coding assistant when you're working in Django.
My point is that they started with vanilla PHP and then had the most successful scaling story of all time -- so if someone else wants to follow in their footsteps then they should go with what is tried and true.
Python is the fastest language in the world if you count in human brain cycles. You should only worry about CPU cycles after you've made your second million.
Honestly, the only kind of re-writes I respect are when someone decides to rewrite something written in the hot new tech and uses 2005-era PHP instead. I recommend solid tools that have been proven to scale to billions of users.
While you're technically correct that they aren't running vanilla PHP, the main takeaway here is that if you want to ultimately want to scale to billions of users then you should probably use vanilla PHP, or maybe Python.