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:
Perhaps it's part of "Security Update Developer Beta 2018-001", which I saw appear on my 10.12 box yesterday. I can't find any info about what's included in it though. :(
(Entry updated Jan 5th)