Yeah, I know. I thought of that right after I posted my comments. So why didn't you post the next line?
"Without admitting that such an act was necessary under the circumstances, it is plain that if the President had in any manner assumed powers which it was necessary should have the authority or sanction of Congress, that on the well known principle of law, 'omnis ratihabitio retrotrahitur et mandato equiparatur,' this ratification has operated to perfectly cure the defect."
Two things jump right out with that one. First, Justice Grier at no time implied that the president broke the law. Second, even if he did believe that then since those actions were not the issues involved in the case then he could not rule on their constitutionality.
Omnis ratihabitio retrotrahitur et mandato equiparatur - every ratification is retroactive and equivalent to a previously granted authority. So why write that "this ratification has operated to perfectly cure the defect"? If it wasn't needed, and no defect existed, why was a retroactive legality necessary?
Second, even if he did believe that then since those actions were not the issues involved in the case then he could not rule on their constitutionality.
What Grier wrote was that depite the real or imagined illegality of Lincoln's actions, the retroactive grant gave Lincoln's action legitimacy. Otherwise the court would have to find Lincoln's actions illegal.