Is "working memory capacity" just another name for word knowledge?

This study is concerned with whether the correlation between complex working memory spans and reading comprehension occurs because the complex spans reflect the capacity of a structural working memory that plays a causal role in comprehension or because a third factor, word knowledge, plays a causal role in both the span tasks and comprehension. If the latter hypothesis is correct, the correlation between word span and reading comprehension should be large when span is tested with low-frequency words but should not occur when span is tested with very familiar words