The added guarantee is less than the provincial government wanted - they went looking for $5.0 billion, apparently - but it will help to defray the escalating cost of the project.
The federal government can justify the money in a number of ways. The most significant justification for helping with the debt is that the provincial government is in desperate financial shape. The federal government had the choice of helping or of almost certainly taking over the provincial government as a bankrupt in short order. As it is, the federal government may well have to take the government in hand in a few years' time anyway.
The $2.9 billion in the new loan guarantee brings the total backed by the federal government to $7.9 billion. At the rate things are going, the project may well cost another $7.0 billion beyond that before things are done.
The federal government will charge one half of one percent as a charge for the loan guarantee.