Try something like that :
sel = bpy.context.selected_objects
act = bpy.context.active_object
for obj in sel:
if obj != act:
bpy.context.sceneview_layer.objects.active = obj #sets the obj accessible to bpy.ops
bpy.ops.object.modifier_add(type='SOLIDIFY')
bpy.context.object.modifiers["Solidify"].thickness = 0.1
bpy.context.sceneview_layer.objects.active = act