Not quite true, there are the altaic languages spoken by Finns, Estonian, Latvians Etc. also Magyar (Hungarian) is a non indo European language as is Turkish. Some studies by some European linguists have found some links between south slavic languages and basque as well as a relationship to celtic tongues. The jury is till out on the basques.
Finnish and Estonian are Uralic languages, not Altaic. Latvian is a Baltic languange, thus Indo-European.
There may be similarities between Basque and South Slavic and/or Celtic, but for all we know, they may well be coincedence. Also, Celts have been living in much of the Iberian pennesuela, so any similarities between Celtic and Basque may well come from borrowing.
My point remains unaffected by your interesting remarks. The Basques were isolated from the mainstream Spaniards even during the 700 year Moorish occupation. Selecting them as representative of the original Spanish genotype, prior to intermingling with Moors, Jews, and others after 1492 is an unproven assumption, thus the confound.
That’s fascinating. I’m debating whether or not I want to order one of those genetic genealogy test kits.