Biden and Putin were in an adversarial setting. They were not there as friends.
Stein, on the other hand, was there as friends of RT, the Russian state propaganda system, and was considered such a good friend of the propaganda machine that they sat her with Putin.
But Stein is explicitly pro-Russia. She has repeated Russian talking points about Ukraine, and only seems to want to handicap Kamala Harris, knowing damn well that she's aiding Trump.