“Reverse-voltage protection” for our regulators is referring to a feature that protects the board in the event that the input power is connected with reversed polarity (i.e. connecting the negative lead on your power source to VIN and the positive lead to ground). The #2868 regulator does not have reverse-voltage protection, so connecting the power with the wrong polarity will damage it.
None of our regulators have any specific circuitry for protecting them if their outputs are connected to the voltages from another source, and we do not generally characterize them for that. You can find some more information on the topic in this post by Jan. Connecting multiple power sources together can potentially damage all of the sources, so you should make sure you either know the inner workings of each source (usually not practical) or connect them with extra circuitry specifically made for connecting multiple power sources together. In some cases, you can get by with just some diodes, and there are also specialized products like this power multiplexer.
Brandon