Add-on script problem

Question

Can anyone tell me why Blender thinks that object type Object does not have attribute modifier_add?

When you check what code Blender use when you manually add any modifier to object, it is this (in this case Bevel modifier):

bpy.ops.object.modifier_add(type='BEVEL')

So I can clearly see object type of Object does have such attribute, right?

But when I do that in my code it puts error message about non-existent attribute of modifier_add...this is my add-on code (it basically adds 3 different modifiers with specific values to any selected object, so that edges look little curved and smooth):

bl_info = {
    "name": "Add Beveling Modifiers To Selected Objects",
    "author": "me",
    "version": (1, 0),
    "blender": (2, 90, 1),
    "location": "View3D > Object > Add Beveling Modifiers To Selected",
    "description": "Add bunch of modifiers that will bevel and smooth edges in selected objects",
    "warning": "",
    "doc_url": "",
    "category": "Object",
}
import bpy
from bpy.types import (
    AddonPreferences,
    Operator,
    Panel,
    PropertyGroup,
)
class OBJECT_OT_addbevelingmodifierstoselected(Operator):
    bl_label = "Add Beveling Modifiers To Selected"
    bl_idname = "object.add_beveling_modifiers_to_selected"
    bl_description = "Add bunch of modifiers that will bevel and smooth edges in selected objects"
    bl_space_type = "VIEW_3D"
    bl_region_type = "UI"
    bl_options = {'REGISTER', 'UNDO'}
# ONLY IF SOMETHING IS SELECTED AND IT IS A MESH
@classmethod
def poll (cls, context):
    return context.object.select_get() and context.object.type == 'MESH'

def execute (self, context):

    # ===== ACTUAL ACTIONS START HERE ===== #
    for ob in bpy.context.selected_objects:
        bpy.context.view_layer.objects.active = ob

        # BEVEL
        ob.modifier_add(type='BEVEL')
        bevel = ob.modifiers["Bevel"]
        bevel.affect = 'EDGES'
        bevel.offset_type = 'WIDTH'
        bevel.width = 0.02
        bevel.segments = 1
        bevel.limit_method = 'ANGLE'
        bevel.angle_limit = 1.48353
        bevel.profile_type = 'SUPERELLIPSE'
        bevel.profile = 1
        bevel.miter_outer = 'MITER_SHARP'
        bevel.miter_inner = 'MITER_SHARP'
        bevel.vmesh_method = 'ADJ'
        bevel.use_clamp_overlap = False
        bevel.loop_slide = True
        bevel.harden_normals = True
        bevel.mark_seam = True
        bevel.mark_sharp = True
        bevel.material = -1
        bevel.face_strength_mode = 'FSTR_NONE'

        # WEIGHTED NORMAL
        ob.modifier_add(type='WEIGHTED_NORMAL')
        weightednormal = ob.modifiers["WeightedNormal"]
        weightednormal.mode = 'FACE_AREA_WITH_ANGLE'
        weightednormal.weight = 20
        weightednormal.thresh = 0.01
        weightednormal.keep_sharp = True
        weightednormal.face_influence = False
        weightednormal.vertex_group = ""

        # SUBDIVISION
        ob.modifier_add(type='SUBSURF')
        subdivision = ob.modifiers["Subdivision"]
        subdivision.subdivision_type = 'CATMULL_CLARK'
        subdivision.levels = 1
        subdivision.render_levels = 1
        subdivision.show_only_control_edges = True
        subdivision.quality = 1
        subdivision.uv_smooth = 'PRESERVE_CORNERS'
        subdivision.use_creases = True
        subdivision.use_custom_normals = False

    # ===== ACTUAL ACTIONS END HERE ===== #

    return {'FINISHED'}


def menu_func_addbevelingmodifiers (self, context):
    self.layout.operator(OBJECT_OT_addbevelingmodifierstoselected.bl_idname)
def register():
    bpy.utils.register_class(OBJECT_OT_addbevelingmodifierstoselected)
    bpy.types.VIEW3D_MT_object.append(menu_func_addbevelingmodifiers)
def unregister():
    bpy.utils.unregister_class(OBJECT_OT_addbevelingmodifierstoselected)
    bpy.types.VIEW3D_MT_object.remove(menu_func_addbevelingmodifiers)
if name == "main":
    register()