King Arthur was mean

Just reading History of the Kings of Britain for the first time.
Geoffrey of Monmouth's Arthur is mean.  He commits near total genocide
of the picts and conquers Scotland, Ireland, then Europe just because
he feels like it.  That was rather surprising.

Did people really used to think that History of the Kings of Britain
was real history?  I know it's fake, but I seem to recall reading that
only recently did people realise it was fake.