- There is zero effect on propensity to work
- The national debt has no secondary effects on the economy
- Changes in taxation do not affect consumption or investment behavior, at any tax level.
Those first two right there are incredibly massive assumptions, and they amount to two of the biggest criticisms of these proposals in the first place. And the third is flat out wrong, with enormous amounts of data available to debunk it.
Moreover, the third assumption ("Increasing government revenue by increasing taxes levied on households does not change household behavior") is actually at odds with their end conclusion! If there were truly no change in household behavior (whether at the margin or in the aggregate), then there could be no primary mechanism for this policy to have an impact in the first place. (And because this study explicitly ignores secondary impacts, without a primary impact, what are they actually claiming to measure?)
So I'm not sure what the value is in a study that basically says, "If we ignore the biggest and most commonly criticized negative effects, the effect of this policy will be positive".