I could have sworn the 16th and 19th 'notes' seemed lower in pitch than the others, but I think that's just a side effect of them having more time before the next hit starts (i.e., the other hits are cut short before the end of the sound effect, making them sound a little higher on average as you don't hear the lower trail-off/echo). Playing the game for myself, I couldn't detect a similar switch in pitch on the shots at all, so yeah, it probably is coincidence.
It's a pity. I thought they would have done some more rigorous testing before releasing the video. Just doing the same audio thing they did there, but starting two hits later in the sequence, or duplicating a pair of notes multiple times would be enough to prove whether it is real or just wishful thinking that is adding pitch to the hits. Or even just let us hear what the rally sounded like without the cuts...