So you can write the new functionalities in the new popular framework and keep the old one running. Why that's a better choice... well it depends on case by case.
However this is a bit like a reverse proxy that load balances many different web apps. You could have different applications written in different languages serving requests to the same url.
Furthermore I don't see anything here that load balancers haven't done since the '90s. Maybe I'm missing something but maybe that's why everybody is puzzled.
It's supposed to be more fine grained than load balancers, and you should be able to scale up and down different parts of the same web app dynamically. That probably didn't come up well in the write-up.
However this is a bit like a reverse proxy that load balances many different web apps. You could have different applications written in different languages serving requests to the same url.
Furthermore I don't see anything here that load balancers haven't done since the '90s. Maybe I'm missing something but maybe that's why everybody is puzzled.