Why does css ":not()" not work when it has no parent selector?

Question

Consider the following example:

:not(p) {
  color: red;
}

<div>
   Red
</div>
<p>
  "p" why red?
</p>

According to MDN docs :not(p) should select only the elements which are not p, but in my example it also selects p, why?

It might be said that :not is pseudo selector so it must be combined with another selector like .test:not(p). Then why does the following work:

  body :not(p) {
    text-decoration: underline;
  }

<div>
  Underlined, because I am not "p"
</div>
<p>
  test
</p>