Yes it is. They even have a page about it: https://sqlite.org/appfileformat.html

It's a format. You can serialise data to it. What about it isn't a serialisation format?

Obviously I wouldn't recommend it for small amounts of data, e.g. for RPC calls. But when you need to store lots of data it's much better than JSON or binary JSON style formats.

But it makes a perfectly reasonable application file format. Coupled with schema evolution and an ORM it's pretty sublime.

It's a data format which can be expressed as a contiguous series of bytes and sent/stored; it's a serialization format.

This is not just nit-picking, it's been used precisely as such:

https://phiresky.github.io/blog/2021/hosting-sqlite-database...