Oh yes, I see your point!
Hmm... if you compare the distribution of L21 (well, I'm not up to date, I don't know how accurate my... well... actually mental map is

) with the spread of the Celts and the Beakers, there is a fairly good correlation with both (even though I wonder why L21 seems to be so rare in Iberia).
However, as I ponder on that now, there is something that makes me think perhaps there is a closer correlation with the Beaker culture, namely that L21 is rather common in Scandinavia as well, wheras the Celts obviously never were in Scandinavia. Now, as far as I know, Beaker material has been found in Jutland. Since this was long before the Germanic ethnogenesis, it would be conceivable how the Germanic peoples inherited their share of L21.
Oh, yes and I could imagine that L21 creates a "false signal" in Britain in regard for that old "acculturalization vs. immigration" dispute, because both the Britons and the Anglo-Saxons had their shares of L21 (assuming the Beaker hypothesis is correct).