No*. Challenges should specify precision requirements.
Challenges that operate with real numbers should include explicit precision requirements- and do so with floats in mind. Unless you have a good reason to require very high precision, you should specify a standard that is well within reason for single-precision floating point numbers, e.g. +/- 0.001 for numbers between -10,000 and 10,000.
What about other representations of real numbers?
Answers should have the freedom to use different representations of numbers. If you specify that an answer can fail due to machine precision, that opens new questions for other representations that have just as much capability to fail. What if I use a half-precision float? What about fractions? Decimal floats? Fixed-point numbers? What standards are these representations held to? What happens when you introduce irrational numbers?
In a comically extreme example, you could implement a solution in such a way that it always returns 0 due to floating point errors even though the math is theoretically correct. It goes against the spirit of solving a challenge if the answer is always wildly incorrect.
When integers are expected
Regarding situations like the referenced challenge, the challenge creator should make this expectation explicit as well. And yes, I think this is a valid thing to expect.
There are cases where a floating point result of a calculation could be close enough to an integer (but not exactly an integer, for instance \$995207\pi = 3126535.0000011\$) to get erroneously rounded to an integer. It is the answerer's responsibility to prove that a floating point error of integer rounding is nevertheless correct for all possible inputs or otherwise work around floating point limitations. Since each individual operation on a float can lose precision, these kinds of almost integers can erroneously become integers and violate the integer precision expectation.
Integers can be held to a much higher standard than floats because they are discrete by their very nature and discrete is what computers excel at.
*Opting in is fine
Challenge writers should still have the option to allow answers to fail due to floating point issues as long as they explicitly state it, but should avoid doing so without a good reason.