Yes, that one. I think he makes a consistent error throughout it, he uses " 'cause" instead of " 'course" as a contraction for "of course (I am)" . Unless it was supposed to be "because I am" but that , if I remember correctly, didn't fit the character's conversation.
I have no trouble looking up obscure words either, but,like you, wish people wouldn't use a thesaurus without checking a words meaning.
Another misuse is definitely when the author meant defiantly, makes me want to send a message to get it corrected. Frustrated beta reader:-)