How is an unprivileged terminal able to signal interrupt to a process launched via sudo as another user?

Question

See for yourself:

% sudo sleep 100 &
[1] 23272
% pkill sleep
pkill: killing pid 23275 failed: Operation not permitted
% fg
[1]  + running    sudo sleep 100
^C
%

How is it that the terminal is able to terminate the privileged sleep process here?

I verified and ascertained two things:

• It is forbidden to signal to processes that belong to any other user, even to nobody.

  % sudo -u nobody sleep 100 &
  [1] 24321
  % pkill sleep
  pkill: killing pid 24324 failed: Operation not permitted
  

  I tried this with SIGTERM, SIGKILL and SIGINT — none get delivered.

• Ctrl+C does send the SIGINT.

  % cat script.sh
  #!/bin/sh
  
  trap 'echo "SIGINT caught!"' SIGINT
  
  while true
  do
      sleep 1
  done
  % ./script.sh
  ^CSIGINT caught!
  

So it does look like we have a contradiction here.