Why would he do that?

So I've liked this guy for 5 years. His sister is one of my best friends. She asked his opinion on me and he said that although I'm really nice, he's not romantically interested or even considered dating me before. When asked if he would ever date me, he said no. She told me all of this and also said she mentioned some other girls so that I'm not singled out too much. I mourned my losses, accepted the rejection, and was happy for the closure. But now, two weeks later, he added me on Snapchat. And we don't normally talk, nor is he really big on social media. It really confused me. If he's not interested, and probably knows that I'm heartbroken, why would he add me on snap? Should I add him back?
Why would he do that?
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