The math.isfinite() docs could be changed to something like, "coerces x to a float if possible and then returns True if x is neither an infinity nor a NaN, and False otherwise."  Or there could be a general note about which functions (most of them) coerce to float (which can fail).

With respect to typing and PEP-484, I don't see a bug or documentation issue.  Types relationships are useful for verifying which methods are available, but they don't make promises about the range of valid values.  For example math.sqrt(float) -> float promises which types are acceptable but doesn't promise that negative inputs won't raise an exception.  Likewise, "n: int=10; len(range(n))" is type correct but will raise an OverflowError for "n = 10**100".