According to "Plant Identification Terminology: An Illustrated Glossary" (Harris & Harris, 2001) [my emphases added]:
A leaf is "an expanded, usually photosynthetic organ of a plant"
- A lamina is the "expanded portion" of a leaf
A petiole is a "leaf stalk" and a stalk is "the supporting structure of an organ, usually narrower in diameter than the organ."
I know you know the definitions here, but I'm using this as evidence that the petiole is often considered separate from the leaf itself.
I do not know of a flora, key, or guide that uses the petiole as part of the measurement.
Since Kew's Plants of the World Online lists this species as having 30 cm long leaves (and assuming they do not include the petiole as is common practice), I would assume your source likewise is not including the petiole.
Disclaimer: I have never seen Ramosmania rodriguesii, so I can't comment from personal experience/knowledge of this specific species. However, if typical measurement practices have been employed, I assume the petiole is excluded in leaf length measurements.