Editorial boards publish opinion pieces. That is the function of the editorial page. Is "we believe that X will be a better president than Y" so different than "we believe that corn ethanol is bad policy" or "we believe that you should send your kids to public universities?"
Yes it is different. Because they go out of their way to claim that they are non-partisan. If you endorsed one candidate and the other one wins, how can we trust your reporting about the winning candidate for the duration of their tenure?
Nobody is claiming neutrality or specific issues like corn subsidies which cross party lines.