Actually not. He may still be in error. It happens. Frankly in this case, with the actions of a Cardinal who says ". . . today we no longer understand ecumenism in the sense of a return, by which the others would 'be converted' and return to being 'Catholics.' This was expressly abandoned at Vatican II.", he may well be. The fact that he has the authority to regulate the liturgy doesn't mean he has the ability to say that heresy is truth. No Pope can do that. Nor did he -- he simply approved what Kasper did -- he may not even be fully aware of the facts, or he may see some strategic import in using an error.
The regulation of liturgy is reserved to the Pope because the liturgy is doctrinal by its very nature. That is, the pope cannot allow a "heretical" liturgy.
Thus, this is NOT an error--it involves infallibility--(faith)--and after the review of the Pope (and others, you can be sure) it's a done deal.