It is immaterial if the runway is grass or hard as far as runway strips are concerned. For a visual runway, code 1, the width is 30m either side of the centreline, and for a visual code 2 runway it is 40m. Straying a bit off-topic but for an instrument runway we hold closer than the edge of the runway strip, it can be 90m from the centreline for a code 4 rwy (ignoring Cat III).
Although you're quite correct in saying you haven't vacated the runway until you are outside this area, in practice providing the acft has vacated the runway
and is keeping going we will clear the next one to land. Sometimes you have too!!
But here I am talking full ATC.
But back to the original post, clear breach of Rule 17(7)(b) in my view.