Yes, oven drying will
not add strength, it is just faster than air drying .... I don't know what would make you think that the oven makes it stronger than an normal open air dry..... but drying is drying....
Sumac is a very porous grass-like plant, and because of that it drys quite quickly, you would probably be better off just letting it dry naturally. I then coated it with about 8 coats of poly to add an extra lamination like film for an added "strength" ( it is really just peace of mind) Just make sure it is nice and stout and you don't put hunter bands on it, then you'll be fine
Cheers - John