I'm not sure if the description says that it can heal soldiers AND reinforce squads, but if it does says that, that is a bug.
The only functions of the Zis-5 medic truck are to heal nearby infantry squads when set down and be a alternate retreat point for your infantry. The only way the Soviets can reinforce other than retreating back to the main HQ is by building an outpost on a point. But make sure that you upgrade "SLIT TRENCH" in the outpost (for 50 munitions) or you will not be able to reinforce around the outpost.
A good tactic for the Zis-5 is placing it behind your lines beside an outpost that has been upgraded and able to reinforce infantry squads. This way if you have a group of infantry in the middle of a losing battle, you can make them retreat to the medic truck where they get healed and can reinforce around the outpost. This is very helpful because your soldiers can heal, reinforce, and be sent back to the front quicker than from the main HQ if they were asked to reinforce there.