Where have you read that ? I can't see that anywhere. Only thing I can find it this :-Flight 5, Row 37 here. If I've read all the information on the Government website correctly, if you've had symptoms of Covid since the flight and then obtain a negative test you no longer have to self-isolate.
"You must continue to self-isolate even if your test result is negative. This is because you might still become infectious".
I've just had my Day 2 test result back and negative but already knew I'd still have to isolate.