Earlier quoted context omitted.
How is it clearly an attempt to play off that slogan? I hadn't heard of the slogan, and 'Eat More Kale' just sounded like it was playing off the pro-Kale zeitgeist of our times. Both vegetarian and paleo diet eaters love Kale (to use two opposite diets as examples), and it's all the rage in trendy urban areas.
If you've heard of "Eat Mor Chickin" you clearly perceive "Eat More Kale" as a vegetarian retort to that. If you haven't heard that slogan... well I don't know what to tell you, what rock have you been living under? EDIT: I think this may be explained by my being from the south and your being from Canada...
But what I really meant is that 'Eat More X' is so generic and easy to come up with that it's not surprising if it occurs to someone independently. For example, 'Eat More Bacon' would make for a good paleo diet T-Shirt.
From what I've read, the chik-fil-a slogan was quite clever and depended on the spelling and context. Simply saying 'Eat More...' on the other hand, is something we all say quite frequently. "Eat more vegetables!"