Mr Forsyth may have exaggerated how fixed adjective order is, but his little nugget is broadly true, and it has delighted people to examine something they didn’t know they knew.
Clearly, then, the discipline of linguistics needs a marketing overhaul, because this is exactly what linguistics consists of: describing the rules, many of them hidden and not obvious, of the human language ability. Given how eagerly word-nerds recently shared this tit-bit about adjective order on social media, the lecture-halls for linguistics classes should be crammed to the rafters.
There are hidden rules not just in grammar, but at every level of language production.
This implicit grammatical knowledge overwhelms, in its intricacy and depth, the relatively few rules that people must be consciously taught at school. But since the implicit stuff is hidden in plain sight, it gets overlooked. It is cheering to see that things like the adjective-order rule can go viral on social media. Perhaps it can make people more likely to associate “grammar” not with drudgery, but with fascinating self-discovery.