A near-perfect sinewave is pretty much what you'd expect (and in fact will receive) at the H-line, since it represents a difference in quantum energy levels, specifically the hyperfine interaction due to flipping the electron-versus-proton spin direction. Any sort of modulation on a 1420-MHz signal would be much more interesting.

What was interesting about the Wow signal is simply how strong it was compared to the expected 1420 MHz background.

Radio astronomy is full of anecdotes like that. My favorite is the 2450 MHz emission that was found to be caused by people in the breakroom opening the microwave oven door before hitting the 'Stop' button.