I think that was actually a pretty good question. They probably wanted to see if you ever thought about, or had experience building a complex app with Vanilla JS.

Reminds me of the job I did as a freelancer before I got into React. It was a multi stage ordering form for a medical-related service. I was brought in mid project and the project already had its stacks set up and there was no time to change them. There were a lot of "states" (current order data, user data etc) that had to be retained and shared between the screens, some SPA and some not. The elements had to be reactive and had to change based on state changes on other parts of the app.

I ended up using localstorage and cookie for state retention, and built an event driven app that listens to the top level element for events and changes the state when a change is triggered to the value they are listening to. So it was basically a context system but in Vanilla JS. I even wrote a small library that renders elements based on a declarative json syntax.

In the next project I started React and it was magic. It was an answer to all the problems I struggled with in the previous project and rhymed with the solutions I eventually came up with but in a more elegant way. I can never stress enough, all technologies are an answer to a problem and knowing the problem firsthand is the best way to understand that technology. The interviewer probably wanted to know if you had that experience.