As I understand the concept and as exemplified in the scriptures:
(1) When a prophet speaks “for God” to either the people on the earth in general, or to the LDS believers who have received the witness of the Holy Ghost about that prophet and about their own testimony of Jesus, then if he does so by the authority of God and under the direct inspiration of the Holy Ghost at that moment in time, then he is speaking “for God.” If he gives his own opinion about a subject without being specifically directed by God to speak under God’s authority and direction on that subject, then he is speaking as “a man” and should be regarded as having given his own opinion. Especially is this true under circumstances where his statement of opinion is not reflective of the united voice of the Quorum of the Twelve Apostles, whose calling and authority includes the calling to protect the doctrinal purity brought forth for the LDS members to gain wisdom from studying and following.
(2) He was speaking his own opinion, as “a man.” He did not have the united sanction of the Quorum of the Twelve Apostles for teaching on that particular subject, nor was his opinion presented for a sustaining vote by the Quorum of the Twelve Apostles or by the members of the church which would need to happen if such a teaching were to need to be viewed as doctrinally correct since it reflected a significant change in doctrine from what Joseph Smith had taught and what the Bible or other scriptures taught on the subject.
That example illustrates the vital importance of having a Quorum of the Twelve Apostles in place, with the keys of authority placed upon them through their having been ordained as such, so that they exercise a role of preserving doctrinal purity through needing to come to agreement if one or more of those particular leaders or the First Presidency has an idea in mind that is a doctrinal change and thus needs discussion and, if it were doctrinally pure through the direct inspiration of the Holy Ghost, needs to be voted upon for the sustaining vote of the Quorum of the Twelve Apostles and of the members of the LDS church in General Conference. If it were a doctrinal matter that reflected a change, it would be presented in that way in that the Quorum of the Twelve would have discussed and voted upon the matter in advance of General Conference, and it would be presented for the sustaining vote of the general membership of the LDS church.