I think the part that makes it a bit confusing is that we are talking about calling run-with-idle-timer with a lower value of SECS from an already executing callback for another run-with-idle-timer. 

On Fri, 21 Aug 2020, 21:49 Eli Zaretskii, <eliz@gnu.org> wrote:
> From: "T.V Raman" <raman@google.com>
> Date: Fri, 21 Aug 2020 12:46:48 -0700
> Cc: raman@google.com,
>     emacs-devel@gnu.org
>
> The whole sentence I cited. How exactly would one specify sec < n?

So you are saying that the sentence is clear, but you don't understand
how such a situation can happen?