There are two different questions in your post:
- The band structure that you are reproducing is not that of a semiconductor. There are bands crossing the Fermi level, so the material is metallic. Looking at the link that you include in the post, there is a plot of the density of states (DOS) right next to the plot you reproduce here of the band structure, and in the DOS it is also clear that the Fermi level is within a group of bands, so the material is a metal.
- Whether the material really is a metal or this is an artifact of DFT is a different question. Semilocal DFT does underestimate band gaps, so if we were to assume that the calculations to generate this band structure are semilocal DFT, it could be that repeating them with a hybrid functional or with many-body perturbation theory techniques a gap opened and the system became a semiconductor. Then again, perhaps the system would stay metallic.
These points are not related to the material being a topological insulator: a topological insulator must have an insulating bulk, and the material you are looking at is a metal.