Wellllll. Despite the title, the paper does not make a claim about string theory. The starting point is the "Witten string field theory" which is a field theory engineered to have properties like string theory. Nothing guarantees that theory is exactly like string theory. In addition, the idea is perturbative in nature, there's no guarantee that perturbative effects are in fact realized in the full quantum theory - exoteric cancellations happen often in field theories with many symmetries. This is two degrees of questionable.

So A) the paper isn't actually about string theory and B) it's not clear that the claim it makes is actually correct for the field theory it supposedly applies to.