Notice how the Isaiah passage is bracketed:
Mosiah 13:34
34 Have they not said that God himself should come down among the children of men, and take upon him the form of man, and go forth in mighty power upon the face of the earth?
Mosiah 13:35
35 Yea, and have they not said also that he should bring to pass the resurrection of the dead, and that he, himself, should be oppressed and afflicted?
Mosiah 15:1
1 And now Abinadi said unto them: I would that ye should understand that God himself shall come down among the children of men, and shall redeem his people.
Abinadi enters Isaiah as a proof text, but the message is that "God himself" will come down and effect the atonement. While most LDS commentaries remain relatively silent on the phrase "God himself," that is nevertheless the driving point behind Abinadi's entire argument (Reynolds and Sjodahl comment directly, but simply reiterate the statement without explanation. Most commentators, save their discussion for the Father/Son text coming in verse 2). This returns very specifically to his counter-attack on the priests of Noah, where he specifically discusses the commandment to have no other gods (see Mosiah 12:35-37, with commentary).
The situation that Abinadi faces is a priesthood that denies the atoning Messiah. They claim to believe in the law of Moses. What Abinadi is doing is using that law against them, and turning their presumptions back on them. Where they presume that they have no other gods, Abinadi is showing them that they have essentially created a new god because they deny the revealed traits of the God of Moses.
For latter day readers, this and the next verses pose some theological confusion because of Abinadi's connection between Jesus and God. There are three points to remember; 1) this is an ancient text and may not have the full understanding of the godhead that has been revealed in the last days, 2) Abinadi's conception of the God of Moses is Jehovah, and understanding that Jehovah is a premortal Christ confirms that Abinadi is absolutely accurate in his descriptions, and 3) the argument linking the atoning Messiah to God is absolutely essential to the point he is making with the priests of Noah.
Textual: It is popular to term Abinadi's comments on Isaiah 53 as a commentary. It really is not. Abinadi paraphrases and expands Isaiah. He uses the Isaiah text as his base, and touches that base more than Nephi and Jacob did when they used Isaiah, but nevertheless is gives his interpretation more than a specific commentary on the verses of Isaiah. In other words, Abinadi uses Isaiah to further Abinadi's arguments - he does not explain Isaiah so that it may be understood.