Although you allege ekke's comment was "much better understandable", it commits the same use of they as singular.
> Others follow a simple pattern - if the light is off, and they have never turned it on yet, turn it on. If it is already on, or they have already ever turned it on, do nothing.
"They" cannot possibly be referring to the group---"they" is referring to each person individually.
If it's referring to the others as a group, it's not a solution to the puzzle.
"Others" is not singular. "they" is a reference to individuals out of a group. "they each" is a common disambiguation, but it doesn't actually matter who turns the light on. The generic he was also used, anyway. This "they" is commonly used to refer to groups of only males, too, and has nothing to do with gendering.
> Others follow a simple pattern - if the light is off, and they have never turned it on yet, turn it on. If it is already on, or they have already ever turned it on, do nothing.
"They" cannot possibly be referring to the group---"they" is referring to each person individually. If it's referring to the others as a group, it's not a solution to the puzzle.
And yet, you found it understandable.