You are right. Els Trocs is in the Pyrenees, just across the French border, so it is not exactly representative of the Iberian peninsula though. This sample is barely a few hundred years older than the Chalcolithic samples from this study. We would still need to figure out exactly how and when M26 got to Iberia.
Actually there is another M26 from MLN in this study that I had missed. It is from the Dolmen de Ansião in Leiria, in central Portugal and is dated 3700-3000 BCE.
Interestingly all the Chalcolithic I2a1a-M26 in the new study are from Portugal (Lisboa or Evora, including Bell Beaker sites) except one from El Mirador near Burgos in Castilla y Leon.
In contrast, the I2a1b-M426 show up in SW, SE, Central and NE Iberia (i.e. in all regions sampled).
At first sight it would appear that they did not spread to Iberia together, as they followed different settlement patterns. But copper metallurgy first appeared in southern Iberia, including the Lisboa and Evora region. M26 later spread to the Atlantic façade of Europe, probably in part with with the Bell Beaker network. The question is where did M26 come from before it got to Portugal? Italy, Greece? Or is it just a coincidence? M26 was already in Portugal just before the Chalcolithic, during the Megalithic period. Therefore, it could be that only G2a-L140 (Z1903) and I2a1b-M426 brought metallurgy from Italy to southern Iberia and M26 just got absorbed in the process?