Often they are but not always, e.g. a value of type other than cell/list can be evaluated at the top-level. How the top-level itself works is an implementation detail.
More fundamentally, being known to something else is not the direction "is a" works in.
Apologies for missing this earlier. At evaluation time, I agree things make a difference. Before that, though, things are merely items in a list. So, yeah, I said it poorly. It is less "everything is a list." And more "everything is in a list." At least at the reasoning level.