This is just silly excuse.

The job of the cofounder is also to anticipate possible risks.

And building your company on an astronomically inefficient technology sounds like a huge risk to me.

Those 1000s of servers are probably a very significant cost with such small technical staff. Just by choosing the right technology for the problem, most of that cost could have been avoided.

Django has nothing special in it that would allow building applications faster than in a lot other frameworks that are also much more efficient.

So it is just a matter of simple choice.

Nobody expects people to write webapps in C++ or Rust. Just don't choose technology that is famous for being inefficient.