Why Mark specified that Zebedee was left alone on the boat

[Giuseppe]

Curiously, the particular of Zebedee being left alone with his servants on the boat is not found in Marcion. I wonder even if the name itself Zebedee is found in Marcion.

But that particular was invented by Mark against Marcion, since it introduces a distinction between fishers (=apostles) and shipmasters (=Marcion):

Then at last they left their boats, and followed Him, understanding that it was He who had begun to accomplish what He had declared. It is quite another case, when he affected to choose from the college of shipmasters, intending one day to appoint the shipmaster Marcion his apostle.

https://www.newadvent.org/fathers/03124.htm

the fishers follow Jesus, while the shipmaster their father (Marcion) doesn't follow Jesus.

But this is an implicit recognition that Marcion was already known by Mark as a shipmaster, too.