This is the first post I've ever heard that peanuts can be poisonous if overeaten, but I know that most tree nuts are. Almonds and Brazil Nuts are high in selenium and can straight up kill you. As few as 6 Brazil Nuts may be enough. Cashews are also slightly poisonous because the fruit they come from is.

You are definitely better of snacking on peanuts than, say, Doritos. It's not that they are a bad food, they just don't have a great macro balance if they are the major component of a diet. From this unvetted comparison they don't seem to be too bad compared to other nuts.

If someone really wanted to get most of their calories from peanuts, they would probably want to supplement with something like pea protein powder and some high-fiber greens (or even beans). This would allow for keeping carbs relatively low while having a more even balance between fat and protein intake. Not quite keto, but not the typical high-carb western diet.