> because they speak an unique old Norse tongue
They don't speak Old Norse, they simply didn't take part in as many changes as in Swedish. Älvdalska still sounds pretty Swedish and we have other North Germanic languages (Icelandic, Faroese) that also kept features that Swedish dropped.
> The runic script was the dominant written language
The runic script is a script and not a language. It's just another alphabet
> flooded by [...] Germanic words
It's a Germanic language in itself, maybe they mean low German loanwords.