This is due to the AddProperty utility method that comes with the CDBPropSet tool class. You use VARIANT_TRUE which is the correct value for OLEDB but it forces the C++ compiler to use this overload because VARIANT_TRUE is a short:

bool AddProperty(DWORD dwPropertyID, short nValue, DBPROPOPTIONS propoptions);

So, just use a true C++ bool, like this:

dbinit[1].AddProperty(SSPROP_INIT_MULTISUBNETFAILOVER, true));

And the utility class will pass a VARIANT_TRUE...