Von Strang appears to have made the road trip outta dodge.
it's always possible a few got out of the inner Hegemony worlds before General K. came knocking on the door. Self preservation can be a powerful motive. Maybe not whole units but selected companies or individuals. I doubt that the SLDF had a complete list of all unit officers, not something that Amaris was likely to share. So after the carnage, a few lances are unaccounted for. Could have been missed in the count, re-assigned off world, etc. The planet is liberated, SLDF garrison installed, no insurgent activity, who cares about a couple of missing Rim Worlders ? 99% of the RWR forces are accounted for and there are more planets and Terra still to be liberated. "Move along, nothing to see here."
Given the self preservation nature of these sorts, I can well see some of them escaping and surviving.