"(c) Even if the mandate may reasonably be characterized as a tax, it must still comply with the Direct Tax Clause, which provides: “No Capitation, or other direct, Tax shall be laid, unless in Proportion to the Census or Enumeration herein before directed to be taken.” Art. I, §9, cl. 4. A tax on going without health insurance is not like a capitation or other direct tax under this Court’s precedents. It therefore need not be apportioned so that each State pays in proportion to its population. Pp. 40–41.
"It need not be apportioned" follows directly from the 16th amendment as this tax is a tax on income and not a direct tax covered by Art. I, §9, cl. 4. and income taxes are specifically exempted of the requirement that they be apportioned.
This is not a tax on income. As your citation says, it is a tax on going without health insurance. The authority cited by the Court is from I.8.1.
Ask yourself this. Suppose the 16th Amendment did not exist. Would this ruling have been any different? My answer is 'no'.