As mentioned by @AndyT's answer, screed doesn't increase load capacity. If anything, it lowers effective capacity since it is itself a load, so there's less capacity to support everything else.
The reason for this is simple: the screed isn't bound to the structural concrete in any meaningful way. As a very rough example, think of an old-school phone book or a thick paper-back dictionary.
If you hold the book along its spine, the pages all sag. Indeed, they almost sag as much as if you were holding each page alone. However, if you hold the book along the top or bottom edges, the pages sag much less, especially near the spine. That's because the spine is literally binding the pages together, making them work in unison to resist the bending load.
Concrete slabs and screed basically behave like the pages when you hold the book along its spine: there's nothing there to help them work together.
That being said, it is possible to make the screen work with the concrete. To do so, you'd need to prepare the slab accordingly: take a pickaxe and/or to the dry concrete, removing the top layer of thin aggregates until you've exposed "raw" large aggregates. This will improve the friction between the concrete and the screed, but it's not enough. You'd also need to drill holes into the concrete and add properly-fixed rebar "hooks" to help transfer loads between the two materials. With this done with great care, you could consider the screed working with the rest of the structure (though not for the slab's and screed's self-weight, since the slab has already deformed to resist that load).
The question is, why would you want to do such a thing? It is hard work, and there's no reason to do it: the screed is usually made of far inferior-quality material, and if you need the added strength, why not just pour an extra-thick layer of concrete in the first place?
I only mention the process because it actually is used on occasion. Just not with screed.
It's how concrete reinforcements are done on aging structures. You calculate the strength needed in a given old beam and then add the necessary steel reinforcement, a layer of high-quality concrete, and additional rebar drilled into the old beam to ensure the old and new concrete work in unison.