Well, I thought the rule was more "no navy odd trousers with odd jackets," period, so further details like a grey jacket wouldn't even enter the equation. I don't have the expertise of a Manton or some others around here, but I've seen enough to tell me that traditionally grey was a much more common odd trouser color than navy, so I'm pretty sure this isn't a new SF rule. I'm sure one can construct a decent fit with navy odd trousers, but in a lot of cases I am not sure it is optimal (with a grey odd jacket, I think I'd prefer cream odd trousers) or that another color might not be as good even if the navy works. YMMV. With clothing, what is worn tends to be a mix of rationales ranging from aesthetics to tradition and most of the tradition, right or wrong, didn't originate in the 21st century on an internet forum.
I don't tend to care for navy odd trousers with odd jackets for a similar reason that I don't own any charcoal odd trousers. I tend to like the bottom half of my outfit to not be darker than the top half. This challenge has showed some pretty cool fits that go against this general pattern and one can definitely do it from time to time, but I think it would be far more difficult if one had to do this on a near daily basis with odd jackets and trousers. One might say the same for navy odd trousers (imagine having to choose only grey or only blue for your odd trousers). I also tend to see navy as a little bit more formal than grey and thus the color seems off to me as an odd trouser (may not help that I don't really love navy flannel even as a suit). I am tempted to try experimenting with RAF blue odd trousers, which I think could work better than navy (probably a pair in fresco for the summer first).