The two are not mutually exclusive.
It helped spawn the Progressive movement (or the New Left), but it's not a facet.
I disagree with that statement. You are giving Reform Judaism credit for original thought, which it does not deserve.
Reform Judaism is not a leader, it is a follower. Leftwing ideas are not spawned there, they are merely assimilated. Leftwing ideas typically originated among university intellectuals and Democratic Party activists and eventually filter down to Reform Judaism.
You are right, the two are not mutually exclusive. Except they are not a facet of the Progressive movement, merely a contributing factor, and a social grouping whose membership often overlaps with that of the Progressive movement.
Reform Judaism does deserve credit for much original thought, whatever else it may deserve or may be said about it. Leftwing ideas have been spawned there or added to, as well as assimilated. And it is not itself a leader, any more than most movements are. But it contributes a fair number of religious leaders to the Progressive movement. Furthermore, it is an important part of the New Left. Look when Jews moved over in large numbers to the Democratic party. The 30s. The New Deal. Who did FDR and the Democrats sway to get that Jewish vote? The Reform synagogues (which were still strong at the time). Prior to that, there were a lot of Jewish Republicans, I'd wager perhaps even the majority of Jewish voters in the 19th century.
Also, the Reform movement also encouraged (while at the same time it mirrored) the liberalization of many mainstream Christian Churches. Not on its own, of course, but as part of the liberalizing religion movement.
That said, I don't approve of a lot of what they do and what they teach. Being accurate though is important in an intellectual discourse.