CFLAT is a tough word to clue. There are no famous orchestral works in that key. It's the enharmonic equivalent of B Major (five sharps), so there's no obvious reason to go to the trouble of writing music in C Flat (seven flats.)
Ms. Weintraub's clue is "Harpist's home key." That's brilliant, and it's mostly true, especially for modern Concert Harps, the kind you see in orchestras. It does raise the question, why are harps tuned to such an awkward key?
To understand, you need to know how harps work. There are seven strings per octave (like the seven white keys on a piano), the strings are color-coded according to their letter names, and there are pedals that stretch the strings, altering their pitch, but only in one direction — sharp.
So, the strings all start in C Flat (all seven notes in the scale are flat.) Pushing, say, the D pedal and locking it down causes all the Ds to rise to D natural. Another notch down, and they all sound as D sharp. Correctly positioning all seven pedals allows you to play in any key and keep the color-to-note-name mapping consistent.
If you started in, say, C instead of C flat, you could still play in any sharp key (G, D, A, etc.), but playing in a flat key (F, B flat, etc.) would require you to drop down to a different (now raised) string. That messes up the color mapping, but critically, it would make that string unavailable for the correct note in whatever key you're in.
Why don't harps have 12 strings per octave so every possible note could be played in any key? For one thing, harps already have so many strings it's difficult to keep track of them all. More importantly, you'd lose the ability for harps to do that effect they're most famous for — the glissandos where strings are strummed to make that familiar shimmering effect. That only works if the notes blend because they're all in one key or one chord.
Fun fact: Harpo Marx, who played beautifully, didn't use a C Flat home key or any normal scale. He had his own unique tuning scheme — not surprising given that he was completely self-taught.