To me a fear biter can become a confident aggressive dog if ......
Say their own circumstances change. We had a farm BC/ american shepherd that (so long ago they weren't even recognized to register)
when she had pups would actively pursue anyone not of the family OUT of the house and property...IF she weren't told not to. Normally she would not do this. But if cornered by a stranger....would bite otherwise.
Also, if a fearful dog is
allowed to bite- simply not called off or taken control of by a handler(the usual is a bit in the butt as someone retreats / leaves) then their confidence will build with each person they are able to 'drive off' and they will stop hiding and just start with the biting.
True guards...generally my encounters have been livestock guardians on or off their animals. Will easily bite when in no other circumstances would they consider it. I got more than a few bites checking lambing ewes for a couple college proffessors who would forget to shut up their dogs. If an owner were around the dog would do nothing of the sort and couldn't even be sicced on someone.
To me fear biting and reactivity are different. Usually I see reactivity as frustration...often dogs restrained will show it....held back, tied back or fenced. Fence fighting is just frustration to me. And some dogs enjoy it.

But take the lead off or the fence away and they are NOT aggressive nor are they fearful.
To me fear biting is instability...may be fixable and may not, depending on a ton of things. Just the presence of their person may stop it.....or a change in socialization til they are comfortable with whatever.