If you look at the API, the publish API allows you to push multiple messages in a single API call, and if that is the batching - publishing multiple messages - then yes, you and others are absolutely correct that there is no cost benefit.
But rather than publishing
{
"messages" : [
{ message 1 }, { message 2 }
]
}
You publish
{
"messages" : [
{ container message containing both, still less than 64k }
]
}
Then you've reduced your operations count by half. Cloud services like this have a per message overhead, for things like auditing and delivery tracking, which is why the pricing model is what it is.
If you are ok with having to unpack and process multiple messages at once as I've described, then it's a way to optimize against the pricing model.
The Spotify graphs from Google's monitoring support the numbers I quoted. They may well be batching but Google is seeing (and billing) the numbers I quoted.
I have 64 1k messages? Ok, I am going to put them inside a single 64k message and reduce my request count by 64x, and profit!