You are right that each character can be one of 16 possibilities, from 0-F, and you are right that if you know one character then then instead of it being 16⁶⁴ = 2²⁵⁶ combinations it is 16⁶³ = 2²⁵² combinations.

My point was that OP had asked what would happen if the person gave you a number of different options for a character - "eg, for the fifth position is either 1, 3, f or a". In that case, instead of that position having 2⁴ possibilities, it instead has 2² possibilities, so the total number of possibilities would reduce from 2²⁵⁶ to 2²⁵⁴. Given that, even if you only had 4 possibilities for every character in the key, you would still be left with 2¹²⁸ possibilities overall, which is still impossible to even consider brute forcing.