One can infer this by reading other comments of the individual who wrote the hymn. Did he claim that Joseph Smith was God? The answer to this is a solid no. If you are aware of one document written by the author of the hymn that equates Joseph Smith with God or states that he is God, you might be able to make your point. I should like to see the other statements that support your interpretation of the hymn. I suppose you are going to make me have to dig out my Mormon book collection in preparation?
At any rate, look at what others (particularly Mormons) say about the hymn. Did the composer tell others that it equated Joseph Smith with God? Did an editorial appear that stated that Joseph Smith was God? The answer is, to my knowledge, no.
Who cares? I'm sure that most Mormons would not say that the hymn contains false doctrine and whether Mormons use the hymn is relevant only to your own faith tradition. Yet, the Mormons themselves are the authority on the true meaning and significance of the hymn. You are interpreting it from the outside looking in rather fhan from the inside. Seriously, you have heard from several Mormons themselves here that they do not equate Joseph Smith with God and do not see the song as doing so. Whether the song is incorrect or not is solely a matter of theology. You really have got to stop projecting your own theological predilections onto a Mormon text. Unless you do that, you never will be able to dialogue with them without misrepresenting what they believe. It really is that simple.
You must come at it with an understanding of LDS teaching. Do they believe that there is a plurality of Gods? Yes. Do they believe that there are more holders of the priesthood than one? Yes. Do they believe that there will be others who will receive great glory? Yes. Do the Mormons believe that people will be endued with attributes of God? Yes. In this context can the meaning of the hymn be found not in your external context that contains beliefs foreign to Mormonism.
Yet, even the Bible is rather direct in stating that people will participate in various attibutes and even in the nature of Deity itself, ruling over angels and the cosmos, sitting on the throne with Jesus and the Father, obtaining the glory of Jesus Christ and taking upon them various attributes of God. But, in no case do these teachings equate individual Christians with God. So is it with this hymn you seem so fond of misrepresenting via your own theological predilections and biases.