Using the event dispatch mechanism is flat-out bigger, anyway. Here’s the interface of the original script (that is, global pub/sub functions taking a name), except that the receiver site no longer needs to look at the .detail property so it’s better:

  let t={};
  sub=(e,c)=>((e=t[e]??=new Set).add(c),()=>e.delete(c));
  pub=(n,d)=>t[n]?.forEach(f=>f(d))

(The nullish coalescing assignment operator ??= has been supported across the board for 4½ years. Avoiding it will cost six more bytes.)

This isn't the same though. With EventTarget, if one of the callback throws, the later callbacks would still get called. With yours the later callbacks don't get called.