The main point of this quote is that if you encounter a player who isn't playing particularly well, rather than getting upset at them, you should try your best to help carry them ("cover what that person lacks"). This is good advice in df, because people often resent being told that they're doing something wrong. Buckle down and be a better player on their behalf. Push dps for the both of you.
What this quote doesn't suggest, however, is that all playstyles are equivalently optimal. Intrinsically, it takes a higher level of player skill to push your dps while keeping everyone alive than it does to just heal and do nothing else. You can play however you like, but you shouldn't harbour any illusions about your level of competence. That was the OP's point, if I understood it correctly.
