Office 365 ProPlus is supposed to get its first Semi-annual (Broad) version in Jan. 2018, and that version will be supported until Mar. 2019. That's 15 months, not 18. My guess is that they are counting from when the version appeared in the Semi-annual (Pilot) Channel.
.. So let me answer that in two different ways, "old" vs. "new" terms:
New Windows 10 releases are initially considered "Current Branch" releases, to be used for piloting. After a period of about four months, we'll declare the release as a "Current Branch for Business" release, ready for broad deployment.
New Windows 10 releases in the Semi-Annual Channel are initially to be used for pilot deployments. Â After about four months, we'll declare that the release is ready for broad deployment.
Regardless of the terms, the 18 months is for the release, e.g. Windows 10 1703, and that 18 months starts from the date that it was released.