I've used it as a staple for years alongside other staples. Try not to get too caught up in only feeding items labeled as staples, or avoiding things labeled as occasional. They are simply recommendations and guidelines to help you make decisions. Unless a food item has near toxic levels of something, most things will be ok.
The goal of those charts is to push the goal of feeding nutrient dense foods, and bok choy fits that bill.
In bok choys case, it's the high level of vitamin a that keeps it from being listed as a staple green. Which isnt really a concern. Bok choy is packed full of a multitude of micronutrients and loads of other goodies for health.
If you go over with us what else you have available, we can help you pick some more items too.
-Brandon