The update function in Elm is single threaded - as javascript in the browser is single threaded. This means that it cannot have a race condition where 2 updates get the same value - the increments will be ‘atomic’.
I would actually say that using a timestamp could result in 2 updates getting the same timestamp. If they were processed in succession very quickly, they might end up getting the same millisecond timestamp. I am not sure though, how quickly in succession Elm can call Time.now, and whether this it is really possible to get the same timestamp out of it.