I assume when a quant question provides two variables, we’re meant to assume the values must be different. BUT, in a comparative question, A was (x+4)(y+3) and B was (x+3)(y+4) and the video solution showed a test case where BOTH x and y were 0 - is this allowed? Do variables only represent distinct values if that’s stated?

different variables imply they must represent different numbers](Do different variables imply they must represent different numbers (as a general rule)?)

No, nothing like that, unless we say that they must be different (such as saying that x \neq y).