Hey, no need to be sorry. I appreciate the search for correctness and especially the reference document.

Here’s what I’ve found.

There is no mention of km/h in section 4, “Non-SI units that are accepted for use with the SI”. It does mention h, making it a “non-SI unit that’s accepted for use with the SI.”

km/h is its own unit separate from h. It’s a unit of speed, derived from km and h.

My gut feel at this point is that km/h could be an SI unit since it’s a unit of speed derived from an SI unit for distance and a non-SI unit accepted for use with the SI for time.

Furthermore, searching the document for mentions of km/h, there’s this bit on page 127, section 2.1, “Defining the unit of a quantity”:

For a particular quantity different units may be used. For example, the value of the speed v of a particle may be expressed as v = 25 m/s or v = 90 km/h, where metre per second and kilometre per hour are alternative units for the same value of the quantity speed.

This paragraph suggests (even though it doesn’t outright say it) that km/h is indeed an SI unit.

I haven’t found anything clearly saying whether km/h is an SI unit or not. Not on that document, not by searching the web. The research above makes me lean towards the idea that it is one.

If you found otherwise, I’d love to compare notes and learn further.