I've looked into his more recent past and he has said he's a regular Dem, not a progressive.
Also worth noting, the term progressive has itself been somewhat hijacked by the activist left since the above tweet came out. Its now come to mean a tidy suite of policies exemplified by the likes of AOC and the squad. This was less so the case for most of the 2010s when ordinary Dems would call themselves progressives, but still adhere to what is today mainstream, corporate Dem orthodoxy. Hilary Clinton for example, insisted she was a progressive when running against Bernie Sanders in 2016. Fetterman would seem to be in the same category given that no one has every mistaken him for an AOC type in the present.
You should perhaps have done some more research then, because there are many more examples than just one tweet from 2016. Even if your point is true about what a progressive was in 2016, what about when he called himself a progressive in 2018? Or what about 2020? For that matter he endorsed Bernie Sanders against Clinton in 2016, so I'm not sure "well Clinton called herself a progressive in 2016" means that much in comparison.
I'm not trying to make the point that Fetterman was ever a
true progressive, whatever that is. For people who looked a little bit closer it's clear he was never
really the potential progressive hero some people thought he might be. But it's indisputable that he
has described himself as a progressive and looked for the aid of the progressive movement when he needed it to get elected.