I'm pretty sure I understand what durability means; the definition is not hard - https://en.wikipedia.org/wiki/Durability_(database_systems)

It's possible I've misunderstood how DELETE mode works. But here's the thing - I shouldn't have to understand how DELETE mode works to know what SQLite setting I need to use to get durability. Unfortunately, the SQLite docs don't clearly say what guarantees each setting provides - instead they talk about about what SQLite does when you choose the setting, leaving the reader to try to figure out if those actions provide durability. And the docs really make it seem like you need synchronous=EXTRA in DELETE mode to get durability, for the reasons explained above.

This is a docs problem; I'm not saying SQLite is buggy.

This may just be a expectations difference then. I would fully expect a developer to read the docs and know how a settings works to know what guarantees it has.