Their tweet has nothing to do with the formula behind their predictions not matching Billboard's formula. It is about the numbers Billboard have reported, as they do not match Rolling Stones' numbers:
"This week, many streams did not count for "Without You", which should've launched to #8 or #9 at the lowest. On Rolling Stone, the song got 12.9 million audio alone, yet Billboard said all streams combined totaled 12.5 million, which is impossible."