The simplest answer is that the folks who introduced R1B into Europe, circa 3,000 bc, were not speakers of an Indo-European language.
2,000 years later (around 1,000 bc), the R1B carrying Western Indo-Europeans (Germans/Celts/Italians) burst onto the scene. They were probably "born" in Central Europe, adopting an Indo-European language from mixing with IE Corded Ware folk to the East. These R1B heavy IE-speaking peoples exploded from Central Europe and conquered the West, South, and North. The process was still not complete during historical times. When the Romans entered Iberia, half the peninsula was IE speaking (Celts) while half was speaking something other (Basque, Iberian, etc.) And to show how language can shift, the Iberian Peninsula ended up adopting neither of these languages, instead they speak an Italic dialect today (obviously).
I guess what I'm trying to say is that the Indo-Europeanization of Western Europe might have been a very recent phenomenon, beginning sometime around 800 bc and ending in modern times. The Basques are a remnant of an earlier language strata. As for their R1B being a newer downstream clad, I think this is a given, considering peoples are always evolving. All it takes is one conquering clad to go through a language shift (R1b males adopting IE in Central Europe) and it could seem like all R1B must have been IE from the beginning. But it might not be the case, if that makes sense.
Of course this is all assumption, but that's what discussion on a forum is all about, right? :grin: