Just to be clear, that's *one* book, which contained lots of fanciful stuff in addition to the Assumption, and its condemnation specified nothing about any erroneous doctrines in it, but only that it was "written and used by heretics".
It isn't the only source for the dogma, or the main one, or necessarily even the earliest one.
I will go with Epiphanius instead of Pius XII. A little closer to the times and arguing from silence is not a bad idea considering the lack of evidence. Let us leave our eternal souls out of this “assumption” and go with those who actually saw/interviewed/wrote down Jesus Christ die rising again and going up to Heaven.