The only thing it proves is that Rondo has been put in a winning situation and has been able to fill a significant role on a championship team. Rubio has not yet had such an opportunity. It's impossible to say whether or not he would rise to such an occasion.
What we can say is that Rubio is putting up excellent numbers and showing great poise and feel for the game as a rookie. I don't think it's a stretch to say that at the same point in his career Rondo could not do the same things that Rondo is doing.
As I've already said, I don't think Rubio is a better player than Rondo right now, but I don't think it's silly to say that they are very similar players who produce in similar ways. Rubio happens to be better earlier in his career than Rondo was (more NBA-ready, you could say), and he's already a considerably better shooter.
Just as we cannot know how great Rubio might be playing if the team around him suddenly transformed itself into the 2007-2008 Celtics, we also cannot know how much better Rondo's performance might have been in 2006-2007 if the team around him suddenly transformed itself into the 2011-2012 T-Wolves.
This Minnesota team is miles ahead in talent over the historic wreck that the 2006-2007 Celtics were. Jeezus, folks - that team lost 58 games!!!! We have no way of knowing how Rubio might have fared if placed in that context.
All we can say is that
1) Right now Rubio is posting excellent numbers for a rookie on a good T-Wolves team.
2) His numbers still trail what Rondo is putting up at the moment for the Celtics.
3) Rondo has been amazingly consistent in posting overall value numbers every season since his rookie year.
The last bit is important because it is a bit disingenuous to dismiss Rondo's accomplishments based on the fact that he has played with "three Hall of Famers". The fact is that NONE of the 'Big 3' are anywhere near the dominant players that they were in their individual peaks and all of them peaked well before they came into Rondo's world.
What they are is 3 still extremely good veteran players who complement each other (and Rondo) to a level far above the sum of their parts. Of the four, Rondo is the one who is young enough to still grow his game and a significant aspect of his game has been to maximize the offensive results of his teammates even as their raw physical talents may be declining. Rondo is a critical part of why Ray and Paul, in their mid-thirties were able to post career highs in shooting percentages last year.
Rubio may end up being a far better player than Rondo. But he has a long way to go before we'll be able to say that for sure. And Rondo is still young and improving his own game.