async doesn't tell you whether the function performs asynchronous operations, despite the name. async is an implementation detail about how the function must be invoked.
As TFA correctly points out, there's nothing stopping you from calling a blocking function inside a future, and blocking the whole runtime thread.
you cannot convert an function that calls async code into a sync function.
u32 / float does not have the problem. It does not "bubble up", unless you want it to.
You can only convert an int to a float with significant caveats. It's not a general trivial conversion. More complicated types may not be convertible at all or behave in all sorts of exciting ways (including having arbitrary side effects).
The point is that none of that is different to async functions. Of course you have to know what to do with them for them to be useful, but there is no requirement for them to "infect" calling code.
The fact that a function can perform asynchronous operations matters to me and I want it reflected in the type system. I want to design my system on such a way that the asynchronous parts are kept where they belong, and I want the type system's help in doing that. "May perform asynchronous operations" is a property a calling function inherits from its callee and it is correctly modelled as such. I don't want to call functions that I don't know this about.
Now you can make an argument that you don't want to design your code this way and that's great if you have another way to think about it all that leads to code that can be maintained and reasoned about equally well (or more so). But calling the classes of functions red and blue and pretending the distinction has no more meaning than that is not such an argument. It's empty nonsense.
"We" don't all agree on this.