I disagree with that so much that it's probably not even worth my time debating. You may as well have just posted that Messi can't do it without Xavi or Iniesta.
Messi's best performances came with Xavi and Iniesta both at their peak and combining incredibly well during Pep's years. He is still incredible but dumbstar is saying just not
as incredible.
There is nothing wrong with that opinion.
That Barcelona team was amazing, when you have an academy like that and so many players that came from that academy you will also be amazing, look at Louis van Gaal's Ajax, them being so young is misleading considering they had spent more time playing with each other and in that way than players from so many other teams in Europe.
I am of the opinion had Ronaldo been at Real Madrid from a young age, graduated from their academy, an academy that played a certain type of football that was consistent with the first team and a type of football that made the most of Ronaldo's ability with players also graduating from that academy in that system on the level of Xavi and Iniesta, the understand he would have with them would be even better and I think he would perform to a higher level for it.
There is nothing wrong with thinking a team is better than an individual, in accepting any player in the world will have a set of conditions that will maximise his performance, such as the manager, the system, the understanding with players around him, the overall team etc.
I don't think dumbstar's opinion was as unfair as you suggest to be honest.
Furthermore he actually posted Messi can still do it, he was just arguing his best performances might have been influenced by the peak of Xavi-Iniesta partnership.