Did it ever occur to you that
のレ イド
and
RAID
mean different things to different groups?
By our standards we look at Western MMOs and say "Okay, it's when a large group of people join up and do a dungeon."
When they look at it, they see Western MMOs using the word raid as an instanced dungeon.
They're both correct. Who cares if it's 5 people doing the instance or 40 people, it's an instanced dungeon. Just accept the term the way they use it. There are so many bigger issues at hand.
EDIT: and to be fair, there really ISN'T a great word for just a 5 man group dungeon besides 'dungeon,' which is incredibly generic and can also be applied to a 10 man group or larger.