Hsuarz ancient Romans called entire region from Rhine to Vistula, Germania. They didnt know differance between germanic people and others, to them all people in that area were "Germans" no matter what was their language. Example, area of ancient Prussia was considered Germania by Romans but people in this ancient Prussia were baltics (Prussians, Galindians, Sudovians) dont you agree? I think that there is no doubt that people in the east of "Roman" Magna Germania spoke some sort of proto-slavic and proto-baltic.
In early times Roman sources always placed slavs and germanic's into same branch called Germani. Only during 5 century did Romans began to differate them. So example in early Roman texts you hear Romans saying that Venedi are germanic tribe but in 5th century they say Venedi are west Slavic tribe. And both examples are according to writers placed in modern east Poland. The thing is that over time they found out Slavic's are different from Germanic's, before that they had few conntacts with them so they thought they were germanics and so they place them on the map together with germanic's into Magna Germania.
Fact that much of Magna Germania was populated by slavic people shouldnt justify their expansionist ambitions. The thing is that Germans belived and some still do that Magna Germania was populated by germanic tribes only, thats why they have these ambitions.
Modern example similar to Magna Germania is America. By America you can think of two different thing. You can say Americans are people from USA or you can say Americans are people on the American continent. Its all a matter of interpretation.