The change you refer to does indeed allow a client snap (such as yours) to request the cursor be locked in position. If the cursor shouldn’t be locked then, ideally, your snap not ask for it to be locked.
However, it is possible to disable the pointer confinement protocols in mir-kiosk to prevent the request being made using drop-wayland-extensions=zwp_pointer_constraints_v1:zwp_relative_pointer_manager_v1 in the configuration file described in Configuring mir-kiosk: a masterclass.