Ermentrud - Is this Elftrude? Or someone else?

According to the Historia Comitum Ghisnensium (History of the Counts of Guines) by Lambert of Ardres, Siegfried 'the Dane' (Sifridus de Dachia), a viking, controlled the area around Guînes in 928, though he never seemed to have been designated as Count.

In about 965 he fell in love with Elftrude (Elstrude) the beautiful daughter of Arnulf (Arnoul) I, Count of Flanders, and she fell pregnant. Siegfried died in 965 and she had a posthumous son, baptised as Ardolf (Ardolph), who became the first Count of Guînes.[1]

This has been accepted by a number of researchers, such as Rösch[2], and is found throughout Internet genealogies, see for instance Darryl Lundy's website,[3] but Baldwin points out that Lambert was writing over 200 years after these events and is not very reliable on other aspects of the history of the Counts of Flanders. Nor are the events confirmed by any other independent sources.[4]

The almost 40 year gap between when Siegfried first is mentioned as rule of Guînes and when he impregnates Elftrude also seems to be unusual suggesting that parts of the story are incorrect. Together these issues cast doubt on the existence of Elftrude and Siegfried.