Apple Watch cardio reading

My watch is telling me my cardio fitness is high I don’t understand as I have long covid, sinus tachycardia and have very low levels of energy so I don’t move about too much. I have a heart rate that swings between 40-178 and I am not able to be physically active as I was before Covid my level of activity would not lift my heart rate to much above 80 so how can my cardio activity reading be high could it be the tachycardia