They weren't fictitious (or not totally), Rothard was the father of Welf I (the ancestor of the elder line of Welf) and they came from the region of Metz, though it is probably incorrect to give Rothard the surname 'von Metz'. As Helmut states they definitely weren't Dukes of Friuli.
There is some information about them in Chapter 9 of Swabia Nobility in Medieval Lands - here
http://fmg.ac/Projects/MedLands/SWABIAN%20NOBILITY.htm
That uses a book by Christian Settipani as a reference, and Settipani refers to Rothard as son of Hardrad but there isn't anymore information on the family, and no wife of Rothard is mentioned.
Birthdates are probably about right, given that Welf's elder daughter Judith married the Carolingian Emperor Louis I in 819, but they can only be rough estimates. Birthplaces are probably fictitious, although there were some Grafs von Altdorf in the family it wasn't for a few generations later and can't be assumed to be a birthplace for any earlier generations.