Sure, you can build a complex API in Flask, FastAPI etc. What I actually meant is for more complex scenarios where you build API's (complex or simple) in addition to using other parts of a web framework that will inevitably come in handy, such as templating, middlewares, schedulers, etc. Then Django has all the batteries included to become productive.
If you discover a hot spot somewhere one can always spin that part off to its own service (which can very well be FastAPI to run it async) or rewrite that particular service in Go or Java. But I would not recommend premature optimization in most cases. First get your product out.
