All trade is by definition positive sum,
Generally true. But requires good information on the part of both parties. With short-term speculation, two people with conflicting information are trading. One is correct, one not.
This is not true for all trades, of course. Someone who believes the price is about to go up may trade with someone who has a better use for the money (rent, electricity, other investments) and a net gain in value is realized but out there on the frontlines, it's red vs black at the roulette table.
Now, to address TA specifically, it doubtless potentially has some value. But that only against others who are trading with bad TA or using other methods that produce poor trading (trading on emotions). However it's not infallible (as I'm sure many TA traders know) and you may be using bad TA (which can only be borne out by time) so there is a real risk of big losses if it's relied on too heavily. And if everyone uses "good" TA, you're sunk (fairly unlikely though).
However, from having monitored this thread for nearly a couple of years now, I have to say, if anyone has good TA, they're mostly keeping it to themselves.
Oh, and if the market's being manipulated and that manipulator knows what will trick most TA, good luck.