Note that this is from the IG report. Horowitz isnt deciding to charge, hes reporting what he was told by the Criminal Division when they reviewed his preliminary report.
US attorneys had an open but inactive file on Manafort. Mueller exploited that.
Other IG reports have similar endings, but sometimes theres no indication of any action or not. When theres no note, something could be up. The bottom line is that misleading the IG by anybody is certainly against the rules and probably illegal.
Thanks much, jjotto.