Hello, video creator here. This is indeed the actual code, and it is written in QuakeC, a language created specifically for the game logic in Quake. The part I'm not sure about is why the creators wanted to add in extra side beams at all.
I think your guess seems plausible, although maybe it was simply an effort to widen the beam to better match the graphics? The lightning effect is rather wide and it may have been surprising for the player if the hitbox ended up too narrow.
I haven't played Quake in the better part of 20 years so I can't comment from first hand experience, but judging by your top-down view at around 41 seconds, the single beam visually seems to intersect with a bunch of enemies while not dealing any damage (probably as a combination of narrow hitboxes on both the beam and the enemies).
