The phrase "having a good rookie season" must have changed since I last used it and is instead being used as a euphemism for playing like crap. That must be it.
I have seen people mention some stats that were objectively good, for example the total yards put him at 6th in franchise history(it's the Bears but still). He only has 6 interceptions which is very good especially for a rookie QB that isn't helped by his coached.