As reported in T80983: Import Images As Planes Imports Alpha as Emission Strength, rBb248ec97769f: Shaders: add emission strength input to Principled BSDF node causes the alpha of the imported image texture to be linked to the new Emission Strength input, instead of the Alpha input. I simply increased the linked input by one.
This version of the addon will only work after rBb248ec97769f. I am not certain if the updating of the Blender subversion inside the addon file is sufficient as this is my first add-on diff.