Even if Russia left it wide open and they had ironclad intelligence to know it, it still would be seemingly too big a risk to commit that many men to what could have been (and still may, frankly) a suicide mission.
I think pure speculation suggests there has to be more to it. That more may simply be that they had to get in front of the US presidential election to try to guarantee more money regardless of winner, but this looks too intricately planned and the Ukrainians appear to be uncharacteristically quiet about what they are doing.