I'm not a doctor but I've done some research on this. This gets a little complicated.
In ALS there is a slow dying of both the "Upper Motor Neurons" (UMN) and "Lower Motor Neurons" (LMN). ALS is different in different people. In some cases UMN are the first to go, in others it's the LMN. To make a "clinically definite" diagnosis of ALS, the neurologist must detect both UMN and LMN symptoms in a few regions. This doesn't happen until the disease has progressed pretty far.
Your doctor was looking at reflexes because heighened or "brisk" reflexes are one of the main symptoms of UMN deterioration. Sometimes LMN shows a decreased reflex, but a more common symptom of LMN is muscle weakening and wasting.
Bottom line is, it doesn't tell you much by itself, but has to be understood within the context of all the other symptoms that the doctor is looking for.