I will try. I tend to think of "The Celts" linguistically as a set of related languages. However, that is an artefact of my own training and background. How the people who spoke those related languages lived can differ radically wherever you find them. For me, to define Celtic is: did they speak a language that is related to those that we have labelled as Celtic? That means that "Celtic" is a convenient shorthand for talking about these languages and does not necessarily have a meaning outside of that.
I have not yet finished reading the linked article so I will see if it may alter my thinking any.
Lorient in Brittany hosts an Interceltique festival; it's a cross-cultural festival that celebrates art, food, language, but it's centered around music and features the best bands from Ireland, Scotland, Wales, The Isle of Man, Cornwall, Brittany, Galicia, Cape Breton, PEI, and New Brunswick.
I've gone several times as an accompanist attached to Irish bands, and it's remarkable to me how I can sit in with other musicians from any of those regions and the idioms, base tunes, time signatures, and ornamentation are so similar. I grew up speaking Irish, and while I can understand Scottish Gaelic, I can't follow anything but a word or two of other other Celtic languages. But with music and dance, things seem much much closer. I have to listen and learn much much more to play with other regional folk music, like Eastern European or even French Canadian (which is sort of close to Celtic and shares many tunes), but with the Celtic countries, I've literally sat in on main stages with no rehearsal and we just all know what to do. It does feel like there is quite a deep shared history there.
Where would the origin of the Celts be according to current archeological finds if you approached it from this linguistic point of view? Atlantic or central Europe?
I like this answer because it's relatable to today's English. English is spoken by many people around the world, but it's not one group of people, it's many many different people and cultures who have come to use English for one reason or another.
But you can still point out who the original English were or at least trace almost 100% of English speakers today as originating from 1600s England's conquest of the world.
The debate about 'The Celts' is that we can't point out where the original language emerged, and the potential range of where it emerged is quite huge stretching from Ireland through central Europe with some claiming it emerged in Turkey even.
What a strange hill to die on. English is the colonial language, and its global success was singularly due to imperialist hegemony. This is taught at the university system level and is in every textbook on the subject. I love a good myth about capitalism just as much as the next exploited worker, but attributing the success of English to trade is equivalent to teaching children myths about Thanksgiving and Washington chopping down a cherry tree.
I have not yet finished reading the linked article so I will see if it may alter my thinking any.