If a purportedly scientific theory CANNOT be disproven through observation, is it really a scientific theory?

Your question falls apart after the "disproven". That is exactly what "theory" is definitively clified as. A series of conjecture, speculation and hypothesis. Even today, there are numerous previously accepted "theories" that are now falling by the wayside with new discoveries in the universe. Theories allow for this adaption and revision. Evolutionary Theory has a long way to go.