I think the diminishing returns of marginal gains would kick in at that point.

Django/Flask is a core part you need to understand in your stack to be productive whereas WSGI/psycopg are random details you don't even need to understand to make use of.

Trying to implement something without a framework can definitely teach one a lot, but it probably will stop him from learning quickly once he understands how server and db driver work. My opinion is that Flask gives one just the right amount of abstractions to be able to iterate quickly and having a full understanding of all processes.