I tried to find them on GitHub. Doesn't seem to be FOSS/OSS either.

Does address a real use-case - might be great as a library or a lightweight alternative to Mermaid.

As SaaS it's a very hard sell.

Yeah, agree. This is the sort of thing you release to build brand awareness and either offer a hosted option as a bonus or integrate into a larger stack. It is not the product. Someone will just make a better OSS option if they don't do it themselves.