Mermaid.js.org links directly to Mermaid Chart, and there is a blurb on the org site that says, "Mermaid + Mermaid Chart... Mermaid Chart is a major supporter of the Mermaid project."

Well, at least they're supporting the project and they're ok with it.

It's not uncommon for companies to piggyback off an open source project's success and mislead people into thinking it's theirs. Seemed an awful lot like that was the case here.

This. On the Mermaid Chart site they say: "By the creators of Mermaid JS" I don't know the details but assuming they're being truthful, there definitely appears to be a tight relationship.