Sorry but how is it serverless? Is it a static HTML file with JS that does the job? Isn't there a server that the request goes to?
However the calendar file gets created out of thin air from the GET parameters alone. There is no state or storage involved. Other calendar apps have a backend with a DB; this one doesn't. Now whether that counts as truly serverless is up to you I guess. It's as close as you could get with a calendar I'd say. Not easy to get the point across in a title. ;) Hope that helps.
FTFY
The key part of serverless is that you don't have a server idling around. Whether or not you use a hosted db is not relevant.