Legends can generally be made to loosely 'fit' history in the way that the bible uses oral tradition from times before the world was officially 'there' and humans learnt to read and write.
Robin Hood could be based on any number of 'bandits' around the time of Richard the Lionheart, who supposedly lived rough in a wood and ambushed the odd taxman. Same with Arthur taken in context with the Romans leaving and the power vacuum they left where the native Britons (so Celts and their druids mainly, but Romano-British too) tried to repel Saxon-type pagan invaders etc.
Anyone remember Arthur of the Britons
back in the early 70s? Apart from the silly posing with swords and stones that's possibly as close as you can get to a reasonably 'historical' re-telling of the Arthur myths which had far more to do with later French knightly stories that owed more to warrior life in the 1200s CE. You could also justifiably argue that, for the possible historical niche, the Grail aspect of the legends are complete and utter cobblers since christianity became distinctly unpopular in Britain almost as soon as the last legion sodded off back to mainland Europe...
So this current Merlin's of a possible magical background and pushed into the knightly take without the christianity - it's fun and pure fantasy because it's all
myth rather than 'quasi-real' or probable legend