I think "it" is formal subject of this sentence.
"It" represents start-up instructions without decompression software.
I examed in this page SPACE ALC seeking example sentence "has confirmed"(http://www.alc.co.jp/index.html), "that" after the word "confirmed" have to bring verb and object in sentence.
Subject "Start-up instructions without decompression" software do not need bring verb and object because what this sentence want to say is "SUBJECT" HAS BEEN CONFIRMED.