monotonically increasing codeword length in optimal (maybe Huffman) code for alphabet with monotonically decreasing frequencies
I misunderstood the phrase "monotonically increasing". It actually means the same thing as "non-decreasing", as explained on p 53 of the book. The phrase is different from "strictly increasing", also defined there, which is what I had thought it meant.

With "monotonically increasing" meaning "non-decreasing", the exercise makes a lot more sense.