TCAS works on closure rate so even if there is no MODE C, if the target aircraft is a few thousand feet below it will have a different closure rate to an aircraft that is 500 ft below, provided all aircraft are in level flight. So some non MODE C will be TAs and some will be proximate or other.
I don't think this is true. Without Mode C (Altitude Reporting) all that can ever be displayed is a TA. The TCAS equipment can't work out a vertical closure rate since it will not know the intruders height, and the horizontal closure rate will be the same whether the intruder aircraft is physically above or below the TCAS equipped aircraft. Won't it ??
All the examples in the bulletin are based on both aircraft having Mode C.