Doesn’t the NYT do a lot of React too? From what I read in other comments Svelte was an experiment.

I think what people mean when they say “at scale” is they want to see a large codebase, with a large team working on it, where Svelte is the primary front-end choice.

The creator of Svelte (Rich Harris) works for the NYT. If you ask him, I'd venture to guess he wouldn't say it's an "experiment" as they regularly use it. I doubt his frontend work there is alone too, so I'd guess it's fairly in-use. But again, that's just my guess.