Visual basic is hardly the first language with a dynamic type, and the DLR is clearly descended from Self, Smalltalk and other dynamically typed languages and started with Iron Python:

https://blogs.msdn.microsoft.com/hugunin/2007/04/30/a-dynami...

VB's variants and COM were not a consideration when conceiving of or designing the DLR, merely a convenient application.

Furthermore, integrating a dynamic type into C#'s type system absolutely built in the work on gradual typing. See their published work: https://www.researchgate.net/publication/221496299_Adding_Dy...