You are correct that a radiant barrier can reflect heat back into a space, including heat produced by radiators. However, the purpose of a radiant barrier is to reflect radiant heat, which is different from convective heat produced by radiators. Radiant heat travels in straight lines and heats objects it comes into contact with, while convective heat is the heat that is transferred through the air. So while a radiant barrier can reflect radiant heat, it may not be as effective at reflecting convective heat produced by radiators.
Discussion
Explain the difference from radiant heat and convective heat
Radiant heat is the transfer of heat through electromagnetic waves, while convective heat is the transfer of heat through the movement of fluids (such as air or water). Radiant heat can be reflected by a radiant barrier, while convective heat cannot be reflected in the same way. Radiators produce heat through convection by heating up the air around them, while a radiant barrier reflects radiant heat.