Earlier quoted context omitted.
The absolute percentage isn’t important. The problem is that when sales tax fails to work as a consumption tax, it gives a tax incentive to be vertically integrated. And there is implicitly a suggestion that encouraging companies to become more vertically integrated is not a desirable property of the tax system.
The percentage doesn't matter, but the example is clearly misleading on the relative tax revenue collected under the example. How does it provide a vertical integration incentive? Sales tax is only applied at the final step (or rebates if applied before that). The main integration incentive is the ability to reduce margin at each step (if needed) and have tighter supply chain control.
Let’s be explicit and instead of talking about sales tax which the article claims sometimes acts like VAT and sometimes like a gross receipts tax and instead use those terms explicitly.
In the gross receipts tax regime, tax is applied at every boundary between businesses and so having fewer of these boundaries (ie being vertically integrated) gives a lower final price with workers receiving equal pay. In this way, the tax system incentives vertical integration.
In the VAT regime, the final price and total tax paid is the same whether or not the business is vertically integrated, and so the tax system doesn’t incentivise one option over the other.
The key point is that we first compare two competing business structures under the same tax system, and it turns out the tax rate doesn’t affect the argument so long as it’s positive, and then we compare how those structures relate to each other in different tax systems (where the rates don’t matter because they don’t change the relationship between the structures that both live under the same rate)