keyof type operator with indexed signature

Question

I was reading through typescript docs and came through this code excerpt:

type Mapish = { [k: string]: boolean };
type M = keyof Mapish;

and they gave the following explanation:

Note that in this example, M is string | number — this is because JavaScript object keys are always coerced to a string, so obj[0] is always the same as obj["0"].

so my question is why number exclusively? why not boolean or object?