When I initially signed up for Notion, I did so thinking the API was already usable. It was used as a selling point for their premium service even though it wasn't implemented. This was a year and a half ago or so. I felt pretty burned by it and killed my subscription. Hopefully this turns out well for the people who've been waiting for it, but that experience left a terrible taste in my mouth.
Isn't that the whole point of Agile MVP development? Sell something that doesn't exist and hope someone buys it? Personally, I don't expect much from it, after working with Airtable's API and hating every second of it.
Indeed, I think that honesty works better; underpromising and overdelivering is a great way to build trust among your initial customer base. Trust that you need to carry people through the inevitable bumps and anticipations of an early-adopter experience.