but entirely appropriate for justices and judges and those seeking such jobs to explain their thought processes, what's relevant, important precedents, the differening views they bring to constitutional interpretation, etc.
yes, it would be inappropriate for a justice or nominee to say something like "this particular potential plaintiff has no case" or anything like that. they should avoid the specifics of any case, particularly a likely upcoming case.
however, they very much can and should give guidance as to relevant constitutional questions, in the abstract, in hypothetical circumstances where they can define the parameters where they can give a meaningful comment. even sitting justices have done this often, rbg has done this, as did scalia. the important thing is to get the caveats right, to define the hypothetical in a way that you can decide.
so, he could answer "*if* there are no offsetting constitutional considerations, there appear to be no limits on presidential pardon power" would be insightful without being committal. alternatively, he could say "i would have to weigh the particulars of the case, considering how to balance the potential tyranny of an aggressive prosecutor against the potential for presidential abuse of a self-pardon." against, insightful without being committal.