"Single-language codebases are the exception rather than the rule in a lot of industries"
And indeed >52% of those Polyglot projects end up failing in more than one way, and are refactored or abandoned. I didn't like that the talk rips on Elixir/Phoenix either, but factually his insights were not inaccurate.
On average it takes around 2 to 3 years to understand why... As it takes time to see why Tacit knowledge is often ephemeral. Tantalizing bad ideas reverberate on the web, and the naive recycle them like facts. YMMV and it usually doesn't matter if you have under 38k concurrent users... and spend $3m/month in transfer fees to AWS. Perhaps your part of the world works differently than mine. =3
And sorry but the other half of your message has nothing to do with what I wrote.