What deductions though? They might just mean agency margin + employer NI. You may still be paying tax + employee ni. They are required by law to advertise this rate but some have been illegally advertising the rate inclusive of employer NI which is probably what that statement is about.
There is no way they will know your tax code there is a reason why jobs aren't posted with take home pay. The take home pay is out of control of the employer so they can't calculate it.