Earlier quoted context omitted.
I think the intended implication goes the other way: "But while we can't safely conclude that beautiful writing is true, it's usually safe to conclude the converse: something that seems clumsily written will usually have gotten the ideas wrong too." In this sense it's similar to "Who speaks bad, thinks bad and lives bad. Words are important!" by Nanni Moretti in Palombella Rossa. Hopefully that's not too much italian…
I’m not sure about that. Does having the right ideas really strongly correlate with having a talent for expressing them eloquently? While clarity of thought facilitates clarity of writing, that’s in principle orthogonal to the right/wrong axis, especially in the ethical sense implied by the Moretti quote. And as the sibling comment correctly observes, the existence of language barriers rather disprove that hypothesis…
It's about the process, not the talent. When you've carefully thought through and refined an idea to understand whether it makes sense yourself, that usually provides a lot of guidance on how you can express the idea eloquently to others. You know the questions they'll have and the answers they'll find satisfying, because you already went through the same process. When you're just tossing out your first half-baked impression, it's a lot harder to communicate it well, although some people do have the orthogonal talent of making it up on the fly.