I think you're focusing on the wrong parts of the comment.
People care about things like long-term support. Postgres 13, from 2020, is still officially supported. Litestream 0.1.0 was the first release, also from 2020, but I can't tell if it is supported still. Worrying about the maturity, stability, and support of an application database is very reasonable in risk adverse projects.
Litestream is just a backup solution. Should probably be compared to a backup solution for postgres that does automated backups over the network etc. That isnt part of postgres.
Besides the question wasnt litestream vs postgres backup apps. It was sqlite vs postgres.
this is essentially the "no one ever got fired for buying IBM" statement. One counter is why buy & manage a rack-mounted server when all you need is a raspberry Pi?
More than once I've started a project with sqlite and then had to migrate to postgres. In my experience it's because of the way sqlite has to lock the whole database file while writing to it, blocking concurrent reads - this isn't a problem in postgres. (There's WAL mode, but it still serialises all writes, and requires periodic explicit checkpointing IME)
You may also find you really want a feature postgres has, for example more advanced data types (arrays, JSON), more advanced indices (GIN inverted index on array members), replication...
Both are mature. There are way more sqlite databases running than postgres. The code base is smaller and has less new features added to it every year. What is unstable with sqlite?