Twice yearly updates and 18 months between major "releases"?
That's not what the article said. It simply said new releases every 6 months, with every release supported for 18 months. There was no "major" or "minor" mentioned.
What this means is, if you are running release n, you can potentially skip n+1, and still have a 6-month period to test and migrate to n+2. You can then take a break while n+3 comes out, and then have another 6-month window to migrate to n+4, and so on.
But you are forced to upgrade every 12 months. You can't do an upgrade every 18 months; that would require migrating from n to n+3 the very day it comes out, which would be madness.