It is? Because more establishment Dems won seats than progressive Dems, afaik.
2018 was a referendum on Trump. According to Rachel Bitcofer, where Dems played it safe and tried not to offense Republicans (aka Trump supporters), they underperformed their numbers. That doesn't mean they lost, but they depressed turnout and made it closer than it had to be. This holds true in the opposite direction too. Most progressives had no qualms calling out Trump and that meant they outperformed their projected numbers even if they didn't win their seats (also, the establishment offering no support for some of these progressives that lost by narrow margins may be the reason progressives didn't pick up more seats).
The thing is, sans negative partisanship (voting against something or someone such as Trump); you've got two strategies that can work.
1. Get out to vote programs. Basically increase turnout by making it easier for your constituents to get to and from polling places; making sure they know and are prepared for elections; and counteracting efforts to depress turnout (Bitcofer mentioned that efforts to bring more Latin voters to the polls in Texas should have been happening for a while now in one of her interviews, I think Majority Report).
2. Galvanize left leaning independents who need a reason to show up (theoretically, that means give them a policy that attracts them so progressives would point to M4A which around 70% of Likely Dem voters supported in the last Morning Consult poll I saw and 55% overall in that same poll, so maybe not just likely independent Dems).
Either way, pointing to seat counting is limiting the discussion. Bitcofer got 2018 nailed down months ahead of anybody because she took a more modern approach to polling and that was one of the key aspects to how she's examined races.
Sidenote: And she's not someone I 100% agree with or anything, so take it how you want. She is pretty confident Warren will beat Biden and Bernie which I'll take though.