It depends on how knex is used. If it gives you a method to run a DB query, and internally checks out a connection, runs the query and releases the connection, then it’s already doing exactly what would happen with a proxy.
If it or you are setting up middleware to start a transaction before every request, or put a checked out connection in the request context, that’s a lot of possible inefficiency. If you really want the convenience this offers, and have request flows that do a lot of other non-DB work, you’ll need a pooling proxy.
If your request handling is very database heavy and does nothing else, then automatic request level connection check out is still efficient, even without a proxy. Even if you have a proxy one real connection is going to be working the entire duration of the request anyway.
Example, using Knex as the connection pooler for your node app, which is connected to your PgBouncer deployment?
I have not considered this a fault, in one way I find it compelling to have a layer of "protection" by using Knex in addition to PgBouncer, but not I'm not that sure anymore.