That should be right and it's working for me. Could you check how birthdays were stored in your vB installation? They should be in the form "06-15-1967" (month-day-year) in the user.birthday table.
You're right - it's a bug. In xF, if the year is not set, it is stored internally as 0 (in xf_user_profile.dob_year). If a year string is given, however, and if that year is '0000', the year will be set to '1900' by the User DataWriter Importer model:
To fix this issue, the importer should specifically look if the string '0000' is given in the source and set dob_year to an empty element accordingly. For example (untested).