Kerry's remark was a bluff, Russia called it in with a bluff, US the calls in that bluff to get out the hole they were in.
No one expected the Russians to call in Kerry's bluff, and the Russians were certainly not expecting the Americans to call theirs in. Its now a lose lose for the Russians as the Americans now have the opportunity for getting troops into Syria under the disguise of securing chemical weapons factories...
Unless this is the scenario that was discussed at the G20.