Confusion!
"taking into account all the cards seen and comprising a dealer's upcard and player's hand, why doesn't a TC=0 strategy correspond to BS?"
Because it isn't intended to be! It ISN'T the definition of BS. A single example should suffice for you to understand the distinction.
Whether 6-deck or 1-deck, the Hi-Lo index for doubling 9 v. 2 is +1. So, if you wanted TC = 0 to correspond to BS, then you would declare that, in both cases, BS for the play is to hit. But it isn't! It's hit for 6-deck but double for 1-deck.
"And I realize they are different things"
Are you sure that you realize that?
"but aren't all indices and the I18 meant to be deviations in strategy from BS?"
Yes. But, again, BS has NO connection to a TC of zero. BS removes the player's cards, the dealer's upcard, and determines the play that maximizes e.v. There is never a mention of the true count, and certainly no assumption of a TC of zero. You need to simply drop the notion from your mind.
Don