I believe the API docs are out of date for updating on collection creation and creating on collection update.
On collectionCreate under id it states:
Specifies the collection to update or create a new collection if absent.
With this input:
input: {
title: 'Special Snares',
descriptionHtml: 'Test',
id: 'gid://shopify/Collection/299687116949'
}
I receive this response:
"userErrors":[{"field":["id"],"message":"id cannot be specified on collection creation","__typename":"UserError"}]
And on the other end, with collectionUpdate, it states:
Specifies the collection to update or create a new collection if absent.
And I receive this response with an id attached:
id must be specified on collectionUpdate