You've been introduced to the facts of any effort I would put out already, on this thread at least.
Your quote doesn't say where the original Celts came from. All people come from somewhere. When Saint Paul went up into Europe and built his churches, he found his people (Isrealites) already there. A thousand years before the Nothern tribes escaped, Israelites took their hats out of Egypt and didn't go home. The only other place they could have gone was Africa and Europe. That matches your time period.
What can a bunch of people do in a vast area in a milenium? A lot wouldn't you suppose?
So we have Old Celts and New Celts. Apparently the common identification as "celt" indicates the two groups were somehow related to the naked eye.
The Northern Kingdom tribes left their captivity around 610ish BC. They were just south of the Causcaus mountains. and northerly was the direction away from their oppressors. It doesn't take a rocket scientist.
But anyway, all this is a real page-turner, but the question remains, who are the descendents of the lost tribes? In order for them to be as the stars in the sky, they would have had to be a major population in any sequential period in history, including this one.
What other candidates?
So now the Lost Tribes of Israel were already scattered before the Assyrian conquest of the Northern Kingdom? Or, at any rate, it's not a problem?
Nothing matches here. Nothing. Not language, not religion, not culture, not timing. Except for that, it's perfect, yes.
So you're saying that, in advance of the collapse of the Northern Kingdom, a wave of Jewish emmigration went to Europe and underwent a language shift to Celtic. They didn't merge with a pre-existing Celtic people; they became the Celts, first wave. They just happened to invent a new language more similar to the ones already there than the one they already had. If they knew how to write, they forgot again until they eventually invented runes. If they were monotheists, they forgot. If they remembered anything of their old pottery and metallurgy styles, they forgot.
Then the Lost Tribes escaped circa 610 BC and became the Celts, second wave. They just happened to invent a new Indoeuropean language similar to the one invented by the Celts, first wave.
So, basically, every time you move a wave of Semitic speakers to Europe, they immediately lose all trace of their language and culture. This includes forgetting for some hundreds of years how to write.
And the evidence for all this is that you don't see the problem with it.