I am trying to understand what's your point here. The documentation clearly states that for what you say it is wrong, it is really true.
So you either put the number of seconds as an integer value, or if you don't, and you use now() helper, that does not returns an int value, but a DateTime.
Instead of passing the number of seconds as an integer, you may also pass a DateTime instance representing the expiration time of the cached item.
https://laravel.com/docs/master/cache#storing-items-in-the-cache