Perhaps we should detect URLSearchParams objects differently (using duck typing detection instead of instanceof window.URLSearchParams, for example) but the solution isn't adding a specific polyfill to Axios (as it'd increase the bundle size and still won't work with other polyfills).

To check whether a data type is iterable look at the method Symbol.iterator.

the promise specification explicitly does not make a distinction

All valid JavaScript code is also TypeScript code. You might get type-checking errors, but that won't stop you from running the resulting JavaScript.