Serverless doesn't mean hard-drive-less, it just means that you, the person deploying the software, don't have to manage servers, only your application which may or may not access files on the filesystem that may or may not be a local disk.

Right, but I think it means this won’t be available for that kind of setup until a service provider adds a SQLite feature.

For Deno Deploy, they went in a different direction using FoundationDB, even though they use SQLite for Deno itself.

(It’s also true that some data centers have machines with no local storage. They use network storage for everything.)

Serverless is just someone else's bash script.

There are quite a few extensions allowing SQLite to run over S3.

I'm trying to figure out how S3 qualifies as "on-disk" AND would be faster than PostgreSQL ...

I believe they do not generally support concurrent use and would be challenging/impossible to use with serverless environments like aws lambda