It doesn't matter what the arguments in Court were. The job of Justices is to weigh the law as it is WRITTEN and SIGNED against the Constitution of the United States. Instead, they deemed the law to say something that was not written, and having so deemed it, found it good.
If I write 2+2=2491.35, and then argue before the Supreme Court that what I really wrote was 2+2=4, it would be as insane for them to declare I had written it correctly as it was to find the mandate was a tax.
What mattered was what was in the law, not the governments arguments about what was in the law.