Since PAF had established local superiority on 27 Feb. IAF was also trying to match us aircraft to aircraft. so whatever they had in the area they made it airborne. There were definitely several mig 21s airborne over kashmir region becoz Sirinagar airforce station is home to 51 Squadron of IAF which is equiped with mig 21 bisons (abhinandan was also from 51 squadron).
i am not saying that it was indeed a mig 21 that shot down MI 17 but there is a possibility becoz the place where it fell is well within indian occupied kashmir some 120 km maybe. since abhinandan was inside pakistani airsapace so it was definitely not him so the point that his weapons were not fired still remains valid.
so that leave us with two possibilities, either a spyder or another aircraft (most probabaly mig 21beco of its proximity home base)