That's not what anyone is talking about.
Take this example from their docs:
@app.get("/items/{item_id}")
async def read_item(item_id: int, q: Union[str, None] = None):
return {"item_id": item_id, "q": q}
You are declaring the types of the parameters, and FastAPI parses and enforces them for you. This saves quite a bit of code, and lets you focus on your business logic. Writing the code this way also allows FastAPI to generate a meaningful description of your API, which can be schema (such as Swagger) that tools can use to generate clients, or it can be documentation pages for humans to read. Any good documentation will still require you to write things, but this gets you further, faster than using something like Flask.
This example[0] takes these concepts even further.
Or just glance at the summary and see that it has nothing to do with @app.post.[1]
FastAPI has also properly supported async routes for longer than Flask, from what I understand.
(I've never personally used FastAPI for anything serious, since I have rarely used Python for anything other than machine learning for the past 5+ years, preferring to use Go, Rust, or TypeScript for most things, but I am aware of it, and seeing its claims misrepresented like that is mildly annoying. FastAPI is far more appealing to me than any other Python web framework I've ever seen, and I've only heard good things about it. Based on my experiences in other languages, their approach to writing APIs is absolutely a good one.)
[0]: https://fastapi.tiangolo.com/#example-upgrade
[1]: https://fastapi.tiangolo.com/#recap