Hmm . . . maybe, maybe not. The line along which I was thinking was that perhaps since Suomi is thought to possibly mean "marsh", there might be a link between "Fennia" and the English "fen", since a fen is a type of marsh. I babelfished 'fen' and found that in German it's 'fenn', so perhaps there was a Germanic language spoken by a people nearby the 'Fenni' who called the Suomi (people from the marsh) the same thing, but using their own version of the word marsh, which was a variation on 'fenn'. Since they would have been conquered/approached first it's entirely possible that Tacticus would have used the Germanic name rather than Suomi.
Am I making any sense? Lol. I'm trying to talk about a time period on which I'm very rusty (although I did study it, archaeologically anyway!).