The issue in this question is one of semantics and one of anatomy. People have been subdividing memory forever (explicit/implicit, declarative/semantic/episodic, procedural/skill based) and so on. Most agree that there are three major areas involved - basal forbrain, mid-lie diencephelon and mesial temporal lobe.
Although I'm not a huge Lezak fan, her point here is well taken. Primary memory (short-term memory) refers to the holding of information in the conscious awareness for a short period of time. It does not mena manipulation per say, just maintenance.
Secondary memory (long-term memory) refers to info that is moved from short term stores and conscious awareness (current) but is retrievable at a later time. Working memory is the current term for STM however, because it correctly implies that the frontal areas do more than maintenence, they are involved in complex attentional capacity for simultaneously storing and processing the information. D'espisito and others have found functional distinctions between inferior/superior aspects of the dorso-lateral pre-frontal cortex involved with maintaining and manipulating info. The main effects of aging are in secondary (LTM) memory. However, primary memory is well preserved and working memory is greatly affected by aging. This is the functional double dissociation that has been called the secondary memory component involved in working memory. Baddeley's model of working memory had three (now four -although it is crazy for reasons too long to go into), working memory has components: the Central Executive - frontal and two "slave sytems" - phonological/articulatory loop & visuo-spatial sketchpad for the temporary maintenance of information. The heart of you question is the finding that the CE undergoes a decline in functioning, but posterior slave systems may not, thus "primary" or temp storage areas could be ok, with a drop in WM attributed a failure in CE at the level of transition to LTM or ability to manipulate info. So, yes, it could explain why digit span is less impacted in DAT, where LNS and other divided attention tests, like trails are awefull.
If you learn anatomy as opposed to terminology, it gets easier.
