This advice applies to Hydrangea macrophylla, the big mop headed and Lace cap Hydrangeas which are fairly common. However, if, like us. you grow the H. paniculata and arborescens types (H. g. Annabelle for example) then these may be cut as hard as you like in February.
The difference is where the flowers are formed, on this type they flower on new wood, so pruning back actually improves the flowering.