Sound waves reflect repeated off the two ends of the tube and interfere with one another. At certain frequencies, standing sound waves are formed, resulting in alternating pressure nodes and antinodes along the tube. This must be the basis for the formation of alternating tall and short flames along the tube.
To figure out the wavelength of the sound wave, we don’t really have to know whether the positions of tallest (or shortest) flame correspond to pressure nodes, or antinodes. We just need to know that the distance between two tallest (or shortest) flames must correspond to half a wavelength of the sound wave.
If you’re interested in knowing which is which, you should watch the next demo.