SAOB says “ibland även väninna”, which means it’s occasionally used as just friend, right?
Sometimes girlfriend is used platonically in Australia, though mostly by older super-straight women.
Incidentally I found a Swedish blog post bemoaning the lack of an appropriate term for a romantic non-married partner older than a girl or boy, same problem as English.
Yes, it's not technically impossible to use it like that, but I honestly doubt even older, super-straight Swedish women would. The meaning of romantic partner is in practice the only possible interpretation.
I don't know why the blogger would write that, though - you can easily use flickvän or pojkvän regardless of age, and there's always partner, sambo, särbo, etc. :)