Yes you are interpreting this correctly. 10/409,992 was simply a temporary means by which the USPTO's collective investigation time/resources is captured again for 12/640344 and used for the forward progress. Thereby not needing to start from scratch which would have been a 12 to 18 months duration.
By capturing the prior investigation time/resources, it is expected that the USPTO's duration decision for 12/640344 is now a 3 months max starting from May/Jun 2012.