No chlorine in pool, nitrates are 30ppm, but pool looks perfect... Do I drain?

I have a 22,500 gallon inground pool. Salt system was just entirely replaced. I have no chlorine in the pool at all. I dropped 2 pounds of shock in and no chlorine. I ran the salt system at 100% and no chlorine. I had a water test and got the following results: chlorine 0, ph 7.2, alkalinity 50, cya 75, calcium hardness 140, phosphates 100ppm, nitrates 30ppm, salt is 3200. Pool store told me not to correct the alkalinity or hardness because of the nitrates. They told me to drain half the pool and refill. Water looks perfect though. If I had to guess, the nitrates came from lawn fertilizer over the last 3 years. The company apparently uses a rotary spreader and I find pellets on the cement around the pool all the time after they come. Needless to say I will stop that going forward. So now do I drain half the pool and retest or do something else?