Not quite true.
Quote from below link: "-- For all the awkward, nervous or frog-in-their-throats public speakers just having a bad day, there's hope. The Supreme Court's decision to uphold the health care law, despite widespread criticism of Solicitor General Donald Verrilli's performance in the courtroom, proves that oral arguments are less about putting on a good show than making viable points and that, more significantly, oral arguments aren't always what seals the deal. Verrilli did, in fact, argue the mandate as a tax, but Roberts's endorsement of this argument reminds us that votes are cast and opinions are written based on much more than courtroom crossfire. Perhaps there's a reason the high court is one of the last venues in American politics still averse to live recordings and modern media technology."
What We Learned: The Tax Argument Cometh
I'm not saying the government didn't argue for it.
In fact, the government argued the tax angle before ALL the lower courts as well. They all rejected it.
What I am saying is that ROBERTS appeared to grab this argument late in the process. I think that is supported by way the other opinions are worded, for the reasons I gave.