Would one of the Mormonism apologists like to explain why the sect in question is not accepted in the LDS fold?
The main reason is the priesthood of God, and line of authority. It’s the same reason other Christian church baptisms and ordinances are not considered valid within the LDS church.
Here’s something from LDS.org that may help you to understand it.
Priesthood Keys
The exercise of priesthood authority in the Church is governed by those who hold priesthood keys (see D&C 65:2; 124:123). Those who hold priesthood keys have the right to preside over and direct the Church within a jurisdiction. For example, a bishop holds priesthood keys that enable him to preside in his ward. Therefore, when a child in that ward is prepared to be baptized, the person baptizing the child must receive authorization from the bishop.
Jesus Christ holds all the keys of the priesthood. He has given His Apostles the keys that are necessary for governing His Church. Only the senior Apostle, the President of the Church, may use (or authorize another person to use) these keys for governing the entire Church (see D&C 43:1–4; 81:2; 132:7).
The President of the Church delegates priesthood keys to other priesthood leaders so they can preside in their areas of responsibility. Priesthood keys are bestowed on presidents of temples, missions, stakes, and districts; bishops; branch presidents; and quorum presidents. A person who serves in one of these positions holds the keys only until he is released. Counselors do not receive keys, but they do receive authority and responsibility by calling and assignment.
Exercising the Priesthood Righteously
Priesthood holders must remember that the priesthood should be a part of them at all times and in all circumstances. It is not like a cloak that can be put on and taken off at will. Any ordination to a priesthood office is a call to lifelong service, with the promise that the Lord will qualify priesthood holders to do His work according to their faithfulness.
Priesthood holders must be worthy in order to receive and exercise priesthood power. The words they speak and their everyday behavior affect their ability to serve. Their behavior in public must be above reproach. Their behavior in private is even more important. Through the Prophet Joseph Smith, the Lord declared that “the rights of the priesthood are inseparably connected with the powers of heaven, and that the powers of heaven cannot be controlled nor handled only upon the principles of righteousness” (D&C 121:36).
In short MHGinTN, the offshoot groups have rejected the valid line of authority of the LDS church. They take upon themselves the rights of the priesthood, which no man should, and they deny the Apostolic calling of those of the LDS church, the reality of temple worship, and the ordinance of the restored gospel of Jesus Christ. You asked, I don't expect you to agree, nor do I intend to argue. Cheers.