One of Bernie Sanders campaign slogans in his first primary campaigns which started in 2015 was “Bernie Beats Trump” - the lack of enthusiasm around Hillary Clinton was palpable.
That slogan is from his 2020 primary campaign.
My quick search suggests that slogan only showed up in 2019.
I think all three of you are right. The official Bernie campaign did not use this as a slogan until the 2020 campaign (so 2019-2020)-- however, it was in use by supporters during the 2016 campaign (source: I was in the crowd of Bernie supporters at the 2016 Democratic National Convention when the guy standing next to me started the chant & it quickly spread. It was so interesting to see how quickly it spread from just one guy.)
Democrats snatching defeat from the jaws of victory is a vintage meme at this point. He may have been the most famous person to say what so many of us expected first, but it just means he paid attention.