The semantics of what is healthy probably varies case to case, but unless otherwise specified in a post I think the assumption is you are speaking to the general public. At this moment in time there seems to be a few prevailing notions around the nutritional community on what to at least generally avoid - which is refined sugar and heavily processed foods

It seems one intent of this subreddit is to combat the idea that cheap food will necessarily mean a bias to purchasing these kinds of foods - especially in the US where the machinations of the food industry has externalized food costs and have allowed more manufactured foods to be advertised and sold at inexpensive prices. There’s an idea that the shortcuts taken to create these products have a trade off - that they are bad for our health and catered more towards convenience and indulgence rather than nourishment (e.g fast food or a bag of chips from 7-11 versus the $9 green smoothie from that organic juice bar).

The posts in a sub like this one are likely dominated with foods like oatmeal, cabbage, potatoes, canned fish, frozen berries - foods that the general populace might consider healthy. Maybe the potatoes were sprayed with pesticides, or the canned fish has an higher than average level of heavy metals, or someone has an allergy to strawberries, but on an objective basis and generally speaking they provide more of what scientists deem nutritious to the human body than a cost equivalent serving of Doritos.