This is in continuation of the post you wrote on my wall.
The employer may be saying that as they would have to apply for extension again after you land in US now. So they would prefer you to get the approval first and then travel to US, so that no action item is required after you land in US.
So legally, you are allowed to travel US even now, but employer is suggesting otherwise to save money and resources (and I can understand their reason and agree w/ it).