I believe this as well. I think she earned the right to claim "died of old age". By that age there could have been a multitude of contributing factors. But she was alert and with it and mobile enough to see out our old PM and welcome in the new one and smile for one final picture. Then passed away 2 days later. That's old age enough for me.
Although I do wonder if the palace would have really released the actual cause of death if it was something a bit more graphic sounding like stroke, brain haemorrhage, heart failure, etc?