They found PMF in a niche (productivity-obsessed email-superusers), which is decent market in size, but raised money and got press coverage like a mainstream so in the end it feels like they kinda failed.

It reminds me of Peloton (though they did go mainstream-ish), or the Snoo (smart baby bassinet)... just because a ton of people email/exercise/have babies doesn't mean the bulk of them are in the market for the hyper-optimized and high-price solution.