-- Module: Mathlib.AlgebraicTopology.SimplicialSet.HoFunctorMonoidal
Goal:
X X' Y Y' Z : Truncated 2
x₀ x₁ :
  X.obj
    (Opposite.op
      (ObjectProperty.FullSubcategory.mk (P := fun (a : SimplexCategory) ↦ a.len ≤ 2) ⦋0⦌ OneTruncation₂._proof_1))
e : Edge x₀ x₁
y :
  Y.obj
    (Opposite.op
      (ObjectProperty.FullSubcategory.mk (P := fun (a : SimplexCategory) ↦ a.len ≤ 2) ⦋0⦌ OneTruncation₂._proof_1))
w :
  homMk (Edge.id y) ≫
      𝟙
        (mk
          ((ConcreteCategory.hom (FC := TypeCat.Fun)
                (snd
                  (X.obj
                    (Opposite.op
                      (ObjectProperty.FullSubcategory.mk (P := fun (a : SimplexCategory) ↦ a.len ≤ 2) ⦋0⦌
                        OneTruncation₂._proof_1)))
                  (Y.obj
                    (Opposite.op
                      (ObjectProperty.FullSubcategory.mk (P := fun (a : SimplexCategory) ↦ a.len ≤ 2) ⦋0⦌
                        OneTruncation₂._proof_1)))) :
              (a :
                  X.obj (Opposite.op { obj := ⦋0⦌, property := OneTruncation₂._proof_1 }) ⊗
                    Y.obj (Opposite.op { obj := ⦋0⦌, property := OneTruncation₂._proof_1 })) →
                Y.obj (Opposite.op { obj := ⦋0⦌, property := OneTruncation₂._proof_1 }))
            (x₁, y))) =
    𝟙
        (mk
          ((ConcreteCategory.hom (FC := TypeCat.Fun)
                (snd
                  (X.obj
                    (Opposite.op
                      (ObjectProperty.FullSubcategory.mk (P := fun (a : SimplexCategory) ↦ a.len ≤ 2) ⦋0⦌
                        OneTruncation₂._proof_1)))
                  (Y.obj
                    (Opposite.op
                      (ObjectProperty.FullSubcategory.mk (P := fun (a : SimplexCategory) ↦ a.len ≤ 2) ⦋0⦌
                        OneTruncation₂._proof_1)))) :
              (a :
                  X.obj (Opposite.op { obj := ⦋0⦌, property := OneTruncation₂._proof_1 }) ⊗
                    Y.obj (Opposite.op { obj := ⦋0⦌, property := OneTruncation₂._proof_1 })) →
                Y.obj (Opposite.op { obj := ⦋0⦌, property := OneTruncation₂._proof_1 }))
            (x₀, y))) ≫
      𝟙 (mk y)
⊢ Eq (α :=
    ((Functor.obj Functor.curry (inverse X Y ⋙ mapHomotopyCategory (snd X Y))).obj (mk x₀)).obj (mk y) ⟶
      Functor.obj
        (Functor.obj (Functor.obj Functor.curry (CategoryTheory.Prod.snd X.HomotopyCategory Y.HomotopyCategory))
          (mk x₁))
        (mk y))
    (NatTrans.app
      ((Functor.obj Functor.curry (inverse X Y ⋙ mapHomotopyCategory (snd X Y))).map (homMk e) ≫
        Iso.hom
          ((fun
                (x :
                  X.obj
                    (Opposite.op
                      (ObjectProperty.FullSubcategory.mk (P := fun (a : SimplexCategory) ↦ a.len ≤ 2) ⦋0⦌
                        OneTruncation₂._proof_1))) ↦
              mkNatIso
                (fun
                    (y :
                      Y.obj
                        (Opposite.op
                          (ObjectProperty.FullSubcategory.mk (P := fun (a : SimplexCategory) ↦ a.len ≤ 2) ⦋0⦌
                            OneTruncation₂._proof_1))) ↦
                  Iso.refl
                    (((Functor.obj Functor.curry (inverse X Y ⋙ mapHomotopyCategory (snd X Y))).obj (mk x)).obj (mk y)))
                ⋯)
            x₁))
      (mk y))
    (NatTrans.app
      (Iso.hom
          ((fun
                (x :
                  X.obj
                    (Opposite.op
                      (ObjectProperty.FullSubcategory.mk (P := fun (a : SimplexCategory) ↦ a.len ≤ 2) ⦋0⦌
                        OneTruncation₂._proof_1))) ↦
              mkNatIso
                (fun
                    (y :
                      Y.obj
                        (Opposite.op
                          (ObjectProperty.FullSubcategory.mk (P := fun (a : SimplexCategory) ↦ a.len ≤ 2) ⦋0⦌
                            OneTruncation₂._proof_1))) ↦
                  Iso.refl
                    (((Functor.obj Functor.curry (inverse X Y ⋙ mapHomotopyCategory (snd X Y))).obj (mk x)).obj (mk y)))
                ⋯)
            x₀) ≫
        CategoryTheory.Functor.map
          (Functor.obj Functor.curry (CategoryTheory.Prod.snd X.HomotopyCategory Y.HomotopyCategory)) (homMk e))
      (mk y))

Tactic:
dsimp