I imagine this is how professional engineers feel about HN folk calling themselves software engineers.
Understanding with complex man made abstractions is much more difficult than plugging data into a thermodynamics calculator
When the GP says "professional engineer", I suspect they mean the big boy official kind [1] that goes to prison if they sign off on a negligent design. It's not a question of difficulty but responsibility and qualifications (though to be clear, the PE is considered more time consuming to prepare for than the BAR exam and it's definitely much harder than plugging numbers into formulas).