If you restrict Venn diagrams to mathematical or abstract concepts that may be true, and the colour academics may already have their own word. But it's a small leap to see the parallel, using the WRF dictionary entry:
a diagram in which mathematical sets or terms of a categorial statement are represented by overlapping circles within a boundary representing the universal set, so that all possible combinations of the relevant properties are represented by the various distinct areas in the diagram
a diagram in which (single)fundamental properties of some phenomenon(in this case light and primaries) are represented by overlapping circles within a boundary representing the universal set, so that all possible combinations of the relevant properties are represented by the various distinct areas in the diagram
It could be a Venn diagram, but unless it has the functionality of such a diagram, it is just a diagram or a picture.
The particular diagram in your image shows the results of adding primary colors of light. In a Venn diagram the area of overlap between two circles represents the intersection of the sets the circles represent. In a way, the overlapped areas in your image represent unions instead.
Where does your second definition come from, Julian?
Edit: I see that you've massaged the WRF dictionary definition to describe the image in the OP. It works until the bit about "all possible combinations."
Edit: The diagram in the OP could be called an additive primary color diagram.
When you are used to very specific distinctions (combinations, additions, unions etc) from the academic field of Venn diagrams where such distinctions are critical, you miss the lay interpretation of "when I combine two primaries I get the colour in the area where both circles overlap and when they are all combined I get white." When you allow for that, the definition is, in my mind, quite , well, fitting The entire image contains the 7 possible combinations of the relevant colours.