well gambling has a clear definition. it is something that you can not predict at all. it is 100% pure chance. for example when you roll a dice or flip a coin there is no way to know what the result of it will be. meaning pure chance.
but when it comes to trading and prices, it is hard to predict but it is not 100% unclear. you can expect certain scenarios to happen with different levels of certainty based on the market situation. and that is a big difference that makes it distinguished from gambling.
If we presume that the absolute majority of traders are losing in the long run, then, to me, this pretty much counts as gambling. I agree that traders do think they can expect certain scenarios to come about more often than others but if they invariably lose in the end, what does it change? When you flip a coin and you don't know anything about your chances, you can also build sophisticated theories why, for example, you should get heads more often than tails. But it doesn't matter, does it? Given that there is a sort of house edge in trading too, we should unmistakably come to the conclusion outlined in OP.