2 ESPs and a 4” color LCD screen (and a keyboard).

Depending on how bright the backlight was, that could eat through battery. And if they were using the wifi for any active communication, that increases power too.

I suspect they wanted it to last the entire weekend with the display always on. The original design probably only had one cell (maybe even smaller battery with built-in protection) and they hurriedly switched to two 18650 cells at the last moment.

They probably went with parallel because that seemed easier, no need to switch to another voltage regulator and charge controller.