The celts are not one group

[The Wizard of BC]

The thing that I see as a very common misconception is that the word "Celt" is used to mean a people or a nation of people. Many customers come into my shop with this misconception.
The English people do speak English and we do sometimes refer to them as The English, but you cannot use the word Celt in the same way.
I think it may stem from the "Neo-Celtic revival" of a few years ago.

The word "Celt" means "One who speaks one of the Celtic Languages". The earliest Celtic language speakers are probably the Hallstatt peoples of the area between the Rhine and Danube rivers. (600-500BC) These are the peoples who gave us the intertwined knot and anthropomorphic designs.
They are distinctively different from those who also spoke a Celtic language but gave us the circles of standing stones. These circles are only found along the western edge of Europe and the British Isles.
These two people are separated by a continent and many hundreds of years. (And these "Celts" are the same peoples that the Romans called "Gauls".)

So when you look at articles like this please remember that "The Celts" have never been one people. They are peoples from throughout Europe and the Isles who spoke a related language. Many Celtic Language speakers are very different culturally and ethnically. Even today.