Yes. And it should be faster. They may forget to create spatial index.

Agree with this. They are re-solving a problem that has been solved better by others before (with R-trees).

They may well be using some data storage where spatial indexing is not possible or standard. Geoparquet is a common one now - a great format in many ways but spatial indexing isnt there.

Postgres may be out of fashion but still an old fashioned postgis server is the simplest solution sometimes.