Wrong. My indie studio is “small”, but we ship an open world game that is only tenable on Unity because our our minspec. I’d switch something not Unity ASAP if something comparable entered the scene.

"Indie," or indepedent, just refers to the studio. Indie studios can work on ambitious titles.

Subnautica is a good Unity example, but there are others

As I have been pointing out for years, and this clusterfuck shows yet again, if you rely on any platform, you are not indie.

Some people also say that indie also means small, otherwise the most indie studios out there would be the likes of Valve and (Microsoft-)Activision-Blizzard.

Independent, i.e. not working with a large publisher, like Microsoft, Sony, etc. That precludes those publishers themselves, of course!

Why would that preclude those publishers, if they also own studios ? (See also : Paradox.)

Also, Valve and Blizzard aren't (weren't ??) publishers.