I don’t actually think Jonah was trying to keep Barnabas in dark about the Fears or that he was only vague in his warnings to “not to cross Mordechai”.

Barnabas just automatically stops listening whenever Jonah starts rambling about anything that sounds too much like some smart science stuff.

Jonah was explaining the Fears and Mordechai’s powers to Barnabas several times. Yet he still couldn’t remember more that the “Mordechai is dangerous or something” part.