Upvote:2
Ashes won't compress over time by any significant amount - everything compressible has been burnt off, leaving just ... ashes. They may leach into ground water however, and then run off. That's a different matter, and would result in land subsidence. Since no land subsidence was noted at the site, let's assume no significant leaching.
So we can take the given volume as being pretty close to the original deposit - whatever the source.
Now my parents' ashes each fit in a box roughly 25cm x 10cm x 15cm, or about 4000 cm3. That is 1 / 2000 of the observed deposit, which suggests the author made the same observation I just did about the volume of ashes from a cremated person.
Modern crematoria are likely more efficient than pagan practices of over 1000 years ago - so that 2000 persons would be the upper limit. However as noted in a comment above by njuffa:
The Islamic traveler Ahmad ibn Fadlan wrote an eyewitness account of the burial of a Viking chieftain. He was burned in a ship together with a thrall girl who was killed just before setting the ship alight. Most of the ashes in such a case would be due to the wooden ship, not from the two corpses aboard.