  The quarks and gluons are confined objects, they cannot be put on the
mass-shell, it is questionable to apply the Landau equation to study the
Feynman diagrams in the QCD sum rules. Furthermore, we carry out the operator
product expansion in the deep Euclidean region $p^2\to -\infty$, where the
Landau singularities cannot exist. The Landau equation servers as a kinematical
equation in the momentum space, and is independent on the factorizable and
nonfactorizable properties of the Feynman diagrams in the color space. The
meson-meson scattering state and tetraquark molecular state both have four
valence quarks, which form two color-neutral clusters, we cannot distinguish
the contributions based on the two color-neutral clusters in the factorizable
Feynman diagrams. Lucha, Melikhov and Sazdjian assert that the contributions at
the order $\mathcal{O}(\alpha_s^k)$ with $k\leq1$ in the operator product
expansion, which are factorizable in the color space, are exactly canceled out
by the meson-meson scattering states at the hadron side, the tetraquark
molecular states begin to receive contributions at the order
$\mathcal{O}(\alpha_s^2)$. Such an assertion is questionable, we refute the
assertion in details, and choose an axialvector current and a tensor current to
examine the outcome of the assertion. After detailed analysis, we observe that
the meson-meson scattering states cannot saturate the QCD sum rules, while the
tetraquark molecular states can saturate the QCD sum rules. The Landau equation
is of no use to study the Feynman diagrams in the QCD sum rules for the
tetraquark molecular states, the tetraquark molecular states begin to receive
contributions at the order $\mathcal{O}(\alpha_s^0/\alpha_s^1)$ rather than at
the order $\mathcal{O}(\alpha_s^2)$.
