If UTF-8 allowed that, then a solution would split words wrong according to UTF-8 rules, but it still would not matter, since the problem statement says that "i

If UTF-8 allowed that, then a solution would split words wrong according to UTF-8 rules, but it still would not matter, since the problem statement says that "it's okay to only support ASCII for the whitespace handling and lowercase operation". So if your solution gets word splitting (or lowercasing) wrong for non-ASCII input, it still gets a pass according to my reading.