My experience with groups which call themselves "christian", but who have little, if any, true connection with authentic Christianity, is that they are quite capable of seizing rulings and canons (such as those from Ecumenical Councils) and putting their own spin on them to suit their own ends, in order to "prove" the "corruption" and "apostasy" of the "early" Church.
Similarly, many modern sects have taken heretical elements (which were, after due consideration, dismissed as heretical, often from more than a thousand years ago), and proclaimed them as "hidden and suppressed teachings of the christian church". (think of the "Da Vinci Code", or "the Gospel of Thomas".) Such people really need to learn their church history. In other words (I am NOT being condescending, believe me!), they are trying to reinvent the wheel.
Therefore, canons which prohibit baptism or administration of sacraments to the dead, must be seen in the light that, even in the earliest Christian period, there were folks and church communitiesthat were doing this sort of stuff, very likely "in good faith", but, who, nevertheless, had to be corrected.