Trende's point (which is an accurate one, if you read the entire decision & dissent) is that Roberts' agreement with Kagan, Ginsburg, Sotomayor and Breyer had nothing to do with the Constitutional power of Congress. All nine justices agreed that Congress has the power to impose a tax on people who do not purchase health insurance. That point is the point that expands the power of Congress
Rather, Roberts' agreement with the liberals (and disagreement with the conservatives) was not related to Congress's power to impose such a tax, but rather whether Congress did so in this particular case. That's obviously an important point within the case itself (and the outcome of the case itself is obviously important, and terrible), but it's not likely to have broader impact on future cases. The point that will likely have impact on future cases is the power of Congress to tax inactivity, and, unfortunately, on that point, all nine justices agreed.
No, all nine justices did not agree that Congress has the power to impose a tax on people who do not purchase health insurance. Where in the text of the dissent do you read such a thing?
From the text of the dissent:
What is absolutely clear, affirmed by the text of the 1789 Constitution, by the Tenth Amendment ratified in 1791, and by innumerable cases of ours in the 220 years since, is that there are structural limits upon federal power—upon what it can prescribe with respect to private conduct, and upon what it can impose upon the sovereign States. Whatever may be the conceptual limits upon the Commerce Clause and upon the power to tax and spend, they cannot be such as will enable the Federal Government to regulate all private conduct and to compel the States to function as administrators of federal programs.Cordially,That clear principle carries the day here. The striking case of Wickard v. Filburn, 317 U. S. 111 (1942), which held that the economic activity of growing wheat, even for one’s own consumption, affected commerce sufficiently that it could be regulated, always has been regarded as the ne plus ultra of expansive Commerce Clause jurisprudence. To go beyond that, and to say the failure to grow wheat (which is not an economic activity, or any activity at all) nonetheless affects commerce and therefore can be federally regulated, is to make mere breathing in and out the basis for federal prescription and to extend federal power to virtually all human activity.
As for the constitutional power to tax and spend for the general welfare: The Court has long since expanded that beyond (what Madison thought it meant) taxing and spending for those aspects of the general welfare that were within the Federal Government’s enumerated powers, see United States v. Butler, 297 U. S. 1, 65–66 (1936).Thus, we now have sizable federal Departments devoted to subjects not mentioned among Congress’ enumerated powers, and only marginally related to commerce: the Department of Education, the Department of Health and Human Services, the Department of Housing and Urban Development. The principal practical obstacle that prevents Congress from using the tax-and-spend power to assume all the general-welfare responsibilities traditionally exercised by the States is the sheer impossibility of managing a Federal Government large enough to administer such a system. That obstacle can be overcome by granting funds to the States, allowing them to administer the program. That is fair and constitutional enough when the States freely agree to have their powers employed and their employees enlisted in the federal scheme. But it is a blatant violation of the constitutional structure when the States have no choice. The Act before us here exceeds federal power both in mandating the purchase of health insurance and in denying non consenting States all Medicaid funding.
[emphasis mine]