I've got two thoughts after reading this. One is that trauma isn't a binary thing, and it can accumulate over time. Some of these mods are dealing with serious events, like the user who was given advice which lead to his wife killing their children. Others got doxxed and receive death threats.
Two, if someone feels upset from getting messages like "you're a piece of shit", I wouldn't say they're an immature child or a weakling. I'd say they could be sensitive, perhaps also very considerate, and in that moment they may worry that another human being is upset and they're responsible for it. They might hurt from the pressure to moderate something that's important to them. Maybe they struggle with social interaction, even if it's online, and experiences like this can be very hurtful.
I don't care why something hurts someone, I care more that they are hurt. Chronic exposure to these things, even if they do seem benign or minor from the outside, absolutely can lead to trauma.
Trauma is a result of exposure to acute or chronic damage to your physical or mental well being. This can occur in a staggering number of ways from person to person. How each of us handles that will vary, but if it leads to a lasting impact, it's trauma.
I'm sorry you experienced what you did. It's arguably worse than what the moderators are describing, but it isn't exclusive to those experiences. It's also not a contest.