Comment on $sigma$-meson: Four-quark versus two-quark components and decay width in a Bethe-Salpeter approach

In a recent paper by N. Santowsky et al. [Phys. Rev. D 102, 056014 (2020)], covariant coupled equations were derived to describe a tetraquark in terms of a mix of four-quark states $2q 2bar{q}$ and two-quark states $qbar{q}$. These equations were expressed in terms of vertices describing the disintegration of a tetraquark into identical two-meson states, into a diquark-antidiquark pair, and into a quark-antiquark pair. We show that these equations are inconsistent as they imply a $qbar{q}$ Bethe-Salpeter kernel that is $qbar{q}$-reducible.