According to us, the 
mark is to be interpreted here as a long accent. The literal interpretation, reproduced quite faithfully in GE, was gradually distorted by the subsequent editions, which made it increasingly different from the original notation.
Compare the passage in the sources »
category imprint: Differences between sources
issues: Long accents, EE revisions, Inaccuracies in FE, Scope of dynamic hairpins,
notation: Articulation, Accents, Hairpins