Hb and RBC are two different concepts.
Hemoglobin is the protein created and stuffed inside each individual RBC.
Anemia can be either decrease in number of RBCs, or concentration of Hemoglobin in RBCs, or both. Either mechanism leads to decrease in oxygen carrying capacity of blood.
Remember, thalassemia can be mutation or deletion of the alpha or beta chain, it could be 2 alpha genes, 2 beta genes, or mixed one alpha and one beta. You can still produce hemoglobin in alpha or beta thalassemia, but the person will either only have the one chain to do so rather than use both chain types, and the resultant hemoglobin is not as good.
I'm not explaining this terribly well, why not look this up?
But yes, actually, RBC count increase can actually lead to decreased hemoglobin concentration within each RBC if essentially the production of RBCs is outpacing the precursors needed to make enough hemoglobin to keep a proper concentration within each.
Conversely, you could have a low RBC count but normal hemglobin concentration within each.
I'm not sure if an elevated RBC count in thalassemia could represent a normal compensation for the issue or if it suggests there is some other issue at work.
Try reading uptodate if you have access for a better discussion than what I've given you.
Bottom line is to understand all the values in a CBC are in fact distinct measures, although often an effect on one can be reflected in another, they are not merely proportionally related nor exactly the same or substitutable. You hear hemoglobin and crit used interchangeably, and that's usually fine, but know that they represent different measures, and they are cases where they don't line up as expected and this is a clue. This is why all the measures exist and are reported, interpreted all together can represent differing pathologies in different combinations.
As a physician, you will need to know how to interpret these studies, and know when to order further tests. That's beyond me explaining here but I wanted to throw out that don't forget the value and having the order for a peripheral smear in your backpocket, it can provide valuable info you wouldn't get from the numbers you get from most other blood tests.
Hope this helps a bit.