The MS-SNTP specification includes a new extended authenticator field, which is apparently used by w32time in latest Windows versions. It's not supported by ntp_signd yet. Latest versions of the specification are here:
A support for ntp_signd was recently implemented in chrony and it can detect MS-SNTP packets using the new extended authenticator field, but there is no way to pass the additional data to ntp_signd. A new command or version of the protocol will probably need to be specified.