I'm betting that using a tally stick as a non-numerical memory aid has more to do with conditioning and the "placebo effect" than with the actual recording of data. If you use a tally stick for numerical recording, you become conditioned and may be inclined to use one in non-numerical contexts. If you think a tally stick is going to help you remember what someone said, you will remember it better.

Also, if Sir Walter was keeping both hands busy with tally sticks, he wasn't consuming Scotch whiskey at a fast enough rate to impair his memory.