Most of what you gathered is true - that is, you will be handed 1500 tasks in the callback function. However,
It will count as one API request and will not be limited in any way
is not true; rather, the Node client library is paginating over the data client side by making multiple requests and appending them to the list, and then it hands them on to your callback function after all pages are fetched. The default page size that’s configured for our client library is 50 items (this is configurable, but note the maximum/enforced limit for page size in our API is 100) so this would create 1500 / 50 = 30 API requests to get this much data.