Sam,
Correct, it's ADS-B Out that is stated for the 2020 deadline, ADS-B In is actually not noted in the doc. The ADS-B In/Out is basically a GPS enable replacement to the current transponder. Many companied have pushed the ADS-B for WX.
ADS-B In/Out will actually provide Traffic (Transponder functionality) and Live Weather. The ADS-B signal consists of a 2 part transmission, FIS-B or Flight Information, and TIS-B or Traffic Information. If all aircraft have an ADS-B In/Out, it really is quite impressive, that all will have WX and Traffic capability. Additionally GPS accurate Traffic which offer much more accurate positions than the typical TCAS.
Basically "any aircraft operating in airspace where a Mode C transponder is required today, will also be required to carry an ADS-B OUT transmitter. The FAA is not mandating ADS-B IN systems with this rule.
The rule does not impact the current transponder requirement – meaning aircraft will continue to be required to carry their transponders in addition to this requirement for ADS-B Out equipage after 2020."