I thought it was quite common to have different factions put together draft opinions as part of convincing folks in the middle. It doesn't mean they have the votes. Then they redraft when they know who wins.
I don't think this means Scalia, Alito and Thomas had the votes, only that they thought it was possible they would have them. Roberts - and possibly others - may have been hold-outs for some time. Roberts wrote his own long and detailed opinion, with some parts being adopted as the opinion of the court and some not - that doesn't happen overnight. Or even in a week.
I don't think the journalists speculating here have much understanding of the process. Consider the sloppy drafting and references just that - sloppy work, possibly by demoralized blowhards who just don't care that much.
FYI - another theory is now floating, which is that Roberts wrote the bulk of both opinions, and the wingers were just sloppy about how they adopted Roberts' alternative decision to a dissent:
See NRO Analysis. So under this reading, they're also just really lazy because they didn't bother to write their own opinion. But, it explains the lack of a chief author to the dissent. This also explains why the more crackpot elements of the dissent are in the tax discussion, not the Commerce Clause discussion. Thomas has to write separately on behalf of 19th century views of the commerce clause, something that shouldn't have been the case if Scalia was drafting.
