That sounds like a lazy journalist writing an article from looking at a map and making things up to embellish it and enlarge their cheque...
There is indeed a fort at Scalspie, on the western side of the bay.
But it's an ancient vitrified fort!!!
There's only stone and rock to be found there.
As I recall, there's at least three of these vitrified Stone Age Forts there.
I don't recall a name on that one (I'd have to dig out some references), but there is Dunstrone Fort to the south, and then Dunagoil Fort still further on.
I don't remember reading of any modern are forts, but I could of course be wrong, or have missed it over the years.
This has all been fairly extensively documented over the years, by the historical society on Bute, and their work dates back some time. There have also been some very detailed modern evaluations to, in more recent years.
Modern era items include a decoy village near Rhubodach ferry terminal on the north west (may be remains, but never found so far), and a pillbox in the area of the modern car park at Ettrick Bay, near the tearoom. But this cannot be found either, having collapsed leaving only some concrete as evidence.
Here's the link needed - hours and hours of reading and detail here (maybe even a fort I missed)...Bute - Background InformationBute Heritage, History, Archaeology &Genealogy
I'm afraid you might find some of the links no longer work - when the original maintainer passed away, his material was eventually lost as no-one there thought to take over and maintain it all properly. Can't say this came as a surprise as we had often spoke of how short-sighted the folk he did this for were. They didn't even accept volunteered help for some sort of fear of liability they had talked themselves into. As a result, many articles and pictures have 'evaporated'.