So this would suggest that the Goidelic languages (Irish Gaelic, Scottish Gaelic, Manx) are derived from the Brittonic languages (Welsh, Breton, Cornish). [1]

I wonder how this affects the different proto-Celtic to modern Celtic languages hypothesis. [1]

This would suggest the following language tree: Breton -> (Cornish / Welsh) -> Irish Gaelic -> (Manx / Scottish Gaelic).

[1] https://en.wikipedia.org/wiki/Celtic_languages

This migration predates the arrival of Celtic speakers by many thousands of years.

There were no Brythonic languages 15,000 years ago.

What are you talking about?