Historic holes are common in old timber
Structural timbers may have been in place for a century or more, giving plenty of time for an infestation to occur and end. The holes remain after the insects have gone.
The amount of damage varies
A few surface holes in sapwood are very different from extensive galleries through a small joist or floorboard. Strength should be judged from the sound timber remaining, not from hole count alone.
Previous treatment can confuse the picture
Old roofs sometimes show drill holes, spray staining or treatment records from earlier decades. That history does not prove whether beetles were active then or are active now.
Avoid treatment simply for reassurance
Historic England specifically notes that treatment is not generally necessary for historic insect damage where the infestation is no longer live. Evidence of current activity should come first.
Useful technical sources
These links provide background guidance. A particular building still needs its own inspection.