Do the english user translate "monk making it his object to let go" as "monk knowing nibbāna", right?
it look like "to let go " and "object" means "target", not "something is known by monk".

Yes, I think the English is a bit ambiguous, and it could be understood to mean either (or both).

The word "object" has many meanings, of which two are:


  
  a person or thing to which a specified action or feeling is directed.
  "disease became the object of investigation"
  synonyms: target, butt, focus, recipient, victim
  "he became the object of fierce criticism"  
  a goal or purpose.
  "the Institute was opened with the object of promoting scientific study"
  synonyms: purpose, objective, aim, goal, target, end, end in view, plan, object of the exercise
  


So an English reader might understand it as the focus of meditation (what you're attentive to), or as the purpose of meditation (what your aim or ambition is).