Is this an unsuitable cassette lockring tool?
I think thats a Shimano freewheel, rather than a cassette. 

A Shimano freewheel needs a tool like a Park Tool FR-1 (or FR-1.2 or FR-1.3), which has 12 splines and a diameter of around 23 mm rather than the Shimano cassette tool (Park Tool FR-5 or its variants), which also has 12 splines but a diameter of around 23.4mm. 

See this link from Park Tool on how to remove it.