When Germani?

I found Allan A. Lund: "De etnografiske kilder til Nordens tidlige
historie" in the library. He's a great debunker; you end up doubting
that there ever were any Germanic-speaking peoples. However, he
maintains that Caesar is the first to distinguish between Gallic and
Germanic, that is around 59 BCE. Does that mean that the ethnogenesis
(or rather glotto-genesis?) of Germanic should be put at shortly
before that year? Did Germanic (as a recently established creole)
suddenly spread on previously Celtic territory?

Torsten

BTW How can Sequani be p-Celtic?