That's a fair point, but psychology is so malleable and that even if people believe that it helps them prepare/digest sensitive info, it will directly help them digest it ... placebo effect is real.
Would they not have seen that in this study if that was the case? It's not like this set of data noone was primed to expect these warnings to help and now they are being primed. Or are you trying to say the effect of trigger warnings is actually negative but because people were primed to get something positive out of them we end up with no effect instead of statistically significant negative effects?