You are probably right to decline if you are not sure.
Although you won't study CIS as part of the payroll qualification HMRC do run CIS as part of PAYE. That means you will need to be familiar with PAYE submissions as CIS is included in one of those (the EPS).
My advice would be to achieve the payroll qualification (it can be quite a lucrative add-on service anyway) so you can then confidently offer payroll AND CIS to your clients.
I hope this helps