I dont see why it wouldnt make sens, looking the same age doesnt mean she is, it seem obvious that Fiona isnt human,she know about Neverwhere, and her inhuman speed if anything is a huge give away.
As for what she could be, possibly Aos si/Daoine si (people of the fairy mound), close to elf and fairy, which would explain her longevity, she's also close to nature and use a bow who look realy elvish to me, or something out of medieval fantasy at the very least (even mc comment about it).
It wouldnt make sens, Fiona was honnest in response of what we can tell her, and what she said about the boy definitly doesnt fit with Orla, who's a girl, but the dog is special for sure just very unlikely to be the boy mentionned by Fiona
He was adopted and said that she was watching him from afar, barely interacting with him, and wanted to give him another chance to live is life, something impossible if she keep him as a pet ... ^^