Maybe I'm just underestimating how widely used spreadsheets are for communicating programming information. I've never seen it, but hey, I've never seen a lot of things.
I've done quite a bit of freelance web development, mostly in the telecoms and infrastructure sectors, and there was often a requirement to integrate with a third party. This "integration" almost always took the form of periodically receiving spreadsheets from them, manually importing them into our database, and then also handling the periodic updates. And the apps I was building only needed to query a few rows each day! Sometimes they'd eventually create an API, but most said they couldn't or wouldn't due to time or complexity. Sheetlabs is my attempt at removing some of these barriers.
Anyway, that was _my_ itch, and this is how I'm scratching it!
So I don't think it's about knowing Excel and knowing what an API is as much as that you can train someone to paste a URL in and then they can build their own reports/worksheets that join in your data.
The idea is cool, the website looks good, and this seems like it could be really useful, but I don't get the sense that the right audience is targeted here.
Also an aside to the Sheetlabs folks, sort of a pet peeve of mine, your site returns a blank white page if JavaScript is disabled, you might want to at least throw a noscript tag in so people don't think you are just down.