A norse farmstead would not have had sewers, no. Until the introduction of a fashion for outhouses in the 18th-19th century (which was still seen by some as an unnecessary luxury in the early 1900's) the typical rural Scandinavian toilet would be a horizontal beam between two trees overhanging a dungheap which was used to fertilize the fields. (I'm assuming Norwegian conditions apply to the other Scandinavian peninsular countries, but may be wrong - Denmark tends to be an early adopter of new fashions, and I think Sweden tends to be more diverse?). We see much the same in urban settings, though outhouses seems to have been more common there - examples dating back to the middle ages have been found in the old town of Oslo. Maybe your source got confused by the excavations at Jorvik/York, where some spectacular finds were made in the still fragrant cesspits of the Norse settlement?

Another possible source of tales of viking sewers is the "secret" escape tunnels some medieval nobles and clergy had made on their properties. I don't have a complete inventory at hand, but for a few examples, there's one at Skálholt on Iceland, another couple in Norway at Dønnes (Dynjarnes) church and the medieval King's Farm at Avaldsnes. These are generally post-viking age, dating to the 1200's. As a curiosity, the one at Avaldsnes was lost, but lived on in popular memory until rediscovered in the 1980's. Tales of ancient secret tunnels tend to stick in the imagination, but details drift over time.

The periodisation for medieval scandinavia can vary a bit depending on country and which subject we're discussing, but generally "dark ages" isn't a thing. After the introduction of bog iron smelting technology and the fall of the roman empire but before the viking age, the period from 400-800 AD is the early middle ages, the germanic iron age or sometimes the migration age; in Sweden 500-800 is often termed the Vendel age.

The viking age starts with the latin records of scandinavian raiders in England and France, though we think similar raiders were present on the baltic trade routes as early as the 600's, and Kaupangs (trading centres) were flourshing in the 700's.

In Scandinavia, this is all part of prehistory. Runic writing existed since at least 200 AD, and contact with the Roman empire spans most of roman history, but since the only written records surviving are later recordings of oral tradition and short runic inscriptions, real "written history" starts with the arrival of literate clerics in the 11th century, which is also the approximate end of the viking age.