Bitcoin is 100% immutable UNLESS it's changed.
Well, what isn't? :-)
The thing is, forks happen all the time in Bitcoin, when two miners find a block at roughly the same time. In most cases these forks are resolved pretty quickly. Sometimes it takes several hours though. The built-in rule is "longest chain wins".
From a technical POV there is no difference between these random forks and a deliberate decision by miners. Calling it an attack is therefore questionable, IMO.
RE: Why Immutability is All That Matters. A Reply to @PSamuelson & a Question to the Developers