I don't think that's true. The Lumia design language is actually the Nokia N8/N9 design language (look at pictures of either of them and this is immediately evident), and that was designed in-house by a team lead by Axel Meyer.
(Also, Frog Design is based in Palo Alto, and their web site doesn't list a Helsinki office.)
(Also, Frog Design is based in Palo Alto, and their web site doesn't list a Helsinki office.)