did you find out?

Yeah, in ruby nil and false equate to false, and everything else (including 0!) are truthy. The zero thing causes some fun bugs for beginners :)

It's a two-dimensional array, and both example test cases support this. Here are the arrays used:

[["cat", "dog"], ["duck", "donkey"]]
[]

The first is a 2x2 array. The second is an empty array, it doesn't have any subsets. Both tests work.

A ; in Ruby is the equivalent to a line break. The author uses it so that the block returns h instead of whatever h[w] += 1 would return. What the .inject and its block does in this solution is usually written like this in Ruby:
each_with_object(Hash.new(0)) { |w, hsh| hsh[w] += 1 }
This is a tiny bit more readable, but does on the other hand take 8 characters more to type.

Ruby is the irritating wildcard in languages when it comes to this kata. Your function will be passed a proc/lambda in Ruby, so sorry if my wording confused you.

It simply ensures that I know the return value in the case of failure.