It sort of seems like "sustainable" means something different to every person who uses the word. Sometimes, it seems like "sustainable" doesn't really mean anything at all.
The way we see it, sustainability, like a lot of other things we often treat as binary, is a spectrum. Here's a real life example: we know that electric vehicles are the way to go in the future, but making more of them requires doing a lot of mining to source the lithium needed for electric vehicle batteries. There's some good there, but also some challenges. It's not cookie cutter simple, or picture perfect, and we're justifiably wary of brands who pretend sustainability can be condensed into a hashtag, a certification or an easy peasy stamp of approval.
Even our best options for a sustainable future are nuanced, which is why we gravitate towards the model of a spectrum, rather than using binary definitions like "good" or "bad" for the environment.