On weather you have read this book? I haven't read it, but from the summary provided, it looks very similar to the Sarmatian connection.

I outlined a bit of the Sarmatian theory in my post, The Narts of the Round Table. The problem with this theory is that it has many large holes in it, such as the basis of the theory which is that mounted Sarmatians migrated to Britain in the 4th-century. The reasoning for this theory is based only on the reference of Arthurian cavalry.

Most likely the legend of the Sarmatians was based on earlier continental Celtic or Germanic traditions of returning valued possessions to the water. Items, particularly weapons, have been found deposited in the waters of Northern Europe that date from before Roman presence. In 1950 at Illerup near Skanderborg in Jutland, Denmark, workers came upon hundreds of metal objects. Archaeologists discovered complete weapons for more than 150 well-equiped soldiers that was thrown in a lake. Excavations discovered almost 500 spearheads, 500 lanceheads, 100 swords, more than 300 shields and about 10 sets of horse-harness gear.

What this proves is that a variety of Celtic and Germanic people made offerings in the form of weapons to whatever water god or goddess they worshipped, and was not an isolated incident found in one particular area as many Sarmatian theorists base the theory on.

Of course it is just one theory.