Gurudev in the Yoga Sutras, it is said that by aparigraha (the concept of non-possessiveness or non-greediness) one gets to know their past lives. Is that true?
Yes. When you do not receive anything from anyone, not even their abuses, then you are most centered.
See, material things are different. When people give you things, it is easy for you to not take it; to deny them. But when people abuse you, then you don’t deny them. In fact, when someone insults you, before they can even complete what they are saying, you take the insult from them.
Someone has not fully abused you yet, they have just begun saying it and you have already taken it from them.
That is why aparigraha is important, which means, neither take anyone's insults nor take their compliments, because a compliment can get into your head and make you arrogant, and an insult can sink into your heart and can cause you intense pain.
So aparigraha is neither taking anything positive or negative that people give. If they give, you just smile at them and not harbor them in your mind.
So, when you don’t worry about other’s opinions of you, that is aparigraha. You don’t take anything. And of course, not sitting and seeing, who is giving what and how much they are giving (in terms of material things) is also aparigraha.