This is my conclusion as well. To wit, an approximate translation of his method into Ruby:

  fizzbuzz = (1..100).to_a.map do |i|
    result = "" # 1:23 initialize a variable
    result += "Fizz" if i % 3 == 0
    result += "Buzz" if i % 5 == 0
    result = i.to_s if result.nil?
    result
  end

The corrected solution would be to change `result.nil?` to `result.empty?`, i.e. checking for "empty string" instead of "null".