Aren't isothiocyanate and thiocyanate ligands monodentate?
The answer to your title question is: Yes, they are monodentate.
But I think you have a misconception about when to use which type of counting: It does not depend on whether the ligand is polydentate or not.
The IUPAC guide states that you use the bis-tris-tetrakis-etc. prefix when you want to avoid confusion:

The  prefixes are ‘di’, ‘tri’, ‘tetra’,etc., for  use  withnames for  simple  entities,  or ‘bis( )’, ‘tris( )’, ‘tetrakis( )’,etc., for names for most entities  which themselves contain multiplicative  prefixes or locants.  Care must  also  be  taken  in  situations when  use  of  a simple multiplicative prefix may be misinterpreted, e.g., tris(iodide) must  be  used  for  $\ce{3I−}$ rather  than  triiodide  (which  is  used  for  $\ce{I3^−}$), and  bis(phosphate)  rather  than  diphosphate  (which  is  used  for $\ce{P2O7^{4−}}$).
Source: https://iupac.org/cms/wp-content/uploads/2018/05/Inorganic-Brief-Guide-V1-3.pdf

So while in the case of thiocyanates and isothiocyanates I don't see that there is any confusion avoided by using bis( ), it is certainly not wrong.