Yeah, surprised this wasn't mentioned; XOR as addition mod 2 (note that they're just talking about XOR here, not bitwise XOR) comes up way more often than it being the most complex binary boolean operation.
The post states at the beginning of the 6th paragraph:
"Some extra treats: XOR can be considered an imparity function. When the number of inputs is even, it outputs zero, while when it is odd, it outputs one. It is also the sum part of an adder, that is, without the carry part. "
