Yes i know this theory very well. It is called Veneti theory and was started by three Slovenes: Jožko Šavli, Matej Bor and Ivan Tomažič. Actualy i have read their the book. It is called Veneti First Builders of European Community. I think many things in this theory are possible but there are also many flaws in that book.
Example in book they want to say that West Slavs have origin in Venezia and that they are very diferent from other Slavs however i disagree with that becouse we are similar to east Slavs.
I belive west Slavs have their origin in modern east Poland west Ukraine and west Belarus. I think that all this similarities between west Slavic nations and west-south Slavic nation comes from fact that many West Slavic tribes went to Alps and Balkan and they populated that place.
I belive Venedi, Sclaveni and Anti are three branches of early Slavs.
Problem is that in ancient times Romans and later Byzantines and Franks usualy mixed up names of many tribes. For example Romans called all people living from Rhine to Vistula Germani, but that wasnt reality becouse half of "Magna Germania" was Balto-Slavic and Iranic and not Germanic. Romans liked to throw various different tribes in to the same basket. Then you have another example; Thracians which are according to some Romans germanic tribe according to others they were hellenic and to another Romans they were special. There were many Veneti tribes in different places and at different times according to Roman sources as well.
Often names were transfered from one ethnic group to another. One good example on how names can be transfered are ancient Rus and modern Russians. Ancient Rus were germanic traders, explorers and warriors, modern Russians are Slavic nation. Then you have French who's name comes from germanic tribe of Franks and so on and on.
I dont say that it isnt possible that we are pure ancestors of Veneti but i have some doubts. I belive name was just transfered from ancient tribe of Veneti to west Slavic people
Magna Germania according Romans.