This is a misunderstanding because I was not specific enough.
I meant that if a model does not even start sampling one needs to set
chains = 1 in order to see all error messages, which will tell you in which line of the model code there is a problem.
It is of course an entirely different matter when the model produces samples. Then one needs multiple chains to see potential scale reduction, check if no chain produces divergences, etc… (I never thought of this part of the workflow as “debugging”)