That's easy. What makes someone a sycophant, by definition, is that their encouragement and flattery is unconditional and completely disconnected from any sort of realistic consideration of your ideas.
You can't judge whether LLM is acting like a sycophant without reading the conversation, and you can't judge whether a human is being an irrational pessimist without having the full context.
Are they a highly intelligent, technically skilled, and socially competent person (probably not if they discuss their ideas with ChatGPT instead of a friend), or do they have a high school diploma, zero practical skills, and have spent the past 20 years smoking weed all day?
You can't judge whether LLM is acting like a sycophant without reading the conversation, and you can't judge whether a human is being an irrational pessimist without having the full context.
Are they a highly intelligent, technically skilled, and socially competent person (probably not if they discuss their ideas with ChatGPT instead of a friend), or do they have a high school diploma, zero practical skills, and have spent the past 20 years smoking weed all day?