The Meltdown reference there does not include Sierra or El Capitan (only High Sierra). Apple had included them in a previous version of the page, but have since removed the references, suggesting they remain unpatched.
There's been some previous HN discussion about it:
https://news.ycombinator.com/item?id=16078414
And The Register claims macOS before High Sierra remains unpatched:
https://twitter.com/theregister/status/949358083431546880