This is a factually inaccurate statement. The research on the topic of PTSD triggers is not especially limited, nor is there any indication that giving people heads up about potentially triggering topics is harmful. Some folks would rather have the warning and be able to sleep the following three nights.
And even if you don't believe that, there is nothing wrong with wanting to avoid content that makes you uncomfortable for whatever reason. People read stories for lots of different reasons, and if someone wants to avoid topics that may provoke strong, negative emotional responses, they should be able to do so. Content warnings are a good thing for the people who want or need them, and at worst a mild inconvenience for those who don't. Nothing wrong with having a little empathy.