people said that about Honey (just a chrome app store solo dev project) and look how much they got acquired for. Don't underestimate them.
This product is very niche. From my own experience, performing a single mildly unorthodox keyboard shortcut in front of an average Joe sort of freaks them out. Getting regular people onboard with something like this would be a total nightmare. This alone probably filters down Superhuman's market dramatically (enough to make a VC-backed 100-employee effort seem absolutely ridiculous).
Regarding the merit of Superhuman's ability to actually save time is another conversation others have argued to death about. I'd be interested in what others have to say about this, but I don't think the discussion has been very productive so far.