You're right, but it's possible they learned these words while reading, and not while writing. But yes, using long or obscure words while writing can often backfire.
I'm pleased to announce I knew trebuchet, because Young Science Girl built one, and her teacher was impressed enough to give her a one-pumpkin pass for a place that sells pumpkins until Halloween, then offers them cheap as ammunition for its full-size trebuchet, which is aimed to hit a junked car. (Our pumpkin missed!) It's great fun to go out there on a fall afternoon. There was talk of adding a catapult, but apparently a full-sized one would be capable of sending the pumpkin beyond the boundaries of their property, so they didn't.
Maryn, who has the model trebuchet tucked away