[BugsJustFindMe]

What, in your view, does sending one markup language instead of another markup language tell you about whether the back-end executed some code or only pretended to?

The front-end display is a representation of what the back-end sends it. Saying "but the back-end doesn't send HTML" is as meaningless as saying that about literally any other SPA website that builds its display from API requests that respond with JSON.