Human brains also perceive speech or writing or what have you using grammar-like structures. In the sense brought up by the article, a grammar is just a structured logical representation of some physical phenomenon, like sound or imagery.
In other words, grammar is how we perceive things, and it makes sense that it can be generalized instead of only being applicable to a specific sensory input.
The specific point about parsing visual input is much more obvious when looking at things like graphical charts, user interfaces, etc.; there's clearly a grammar of some form involved in, say, determining which button to press on my phone's on-screen keyboard to create the letter 'b', and further involved when determining what a "button" is or what a "keyboard" is or what a "letter" is. Hell, that seems like the same process that lets me figure out what a "screen" is and what a "phone" is. Eventually we go from "type the letter 'b'" to "move your right thumbtip to this position" (and even that can be broken down further).
In other words, grammar is how we perceive things, and it makes sense that it can be generalized instead of only being applicable to a specific sensory input.
The specific point about parsing visual input is much more obvious when looking at things like graphical charts, user interfaces, etc.; there's clearly a grammar of some form involved in, say, determining which button to press on my phone's on-screen keyboard to create the letter 'b', and further involved when determining what a "button" is or what a "keyboard" is or what a "letter" is. Hell, that seems like the same process that lets me figure out what a "screen" is and what a "phone" is. Eventually we go from "type the letter 'b'" to "move your right thumbtip to this position" (and even that can be broken down further).