None of my old guide books (from the last millennium ;-) makes a mention of that structure, so my guess is that somebody had the following thoughts:
"Najera is connected with the Roldan/Ferregut legend in which a 'poyo' is mentioned. We have a half fallen shepherds/vineyard hut conveniently just beside the Camino, so why don't we restore it and call it 'Poyo de Roldan'."
Even if there is some historic truth in the original legend, I seriously doubt that the original poyo de Roldan would have survived and be correctly remembered all these centuries.
Poyo has been translated as bench or platform. Perhaps in this instance the reference is to the flat-topped hill on which that rather modern built conical rubbish storage facility sits. The Wikipedia article infers that - "...Since then the hill is called Podium or Poyo de Roldán".
The battle or duel between Roland and Farragut is depicted in carving elsewhere on the Camino Frances. See for example this capital block in Estella
or this carved capital on the church facade in Villamayor de Monjardin. For a good overview of the importance of the Roland/Farragut or Roldán y Ferragut legend on the CF in the Estella area see this Spanish account .
Notes from my encounter with the solidly built historical marker:
I passed the monument to the fight between Roldan (Roland) and a Syrian giant, Farragut. The descriptive sign said in English that the nine foot giant had a nose span the width of four men. As I mentally geared up to watch for his descendants with huge noses, I read the description in Spanish, which spoke of a span from hand to nose of four men, so I am just watching for long arms.
The "Olifant", the Horn of Roland, is kept in the Cathedral Museum (the one underneath) in Santiago de Compostela. Years ago it was on display but not the last time we were there. When I asked, they did say it was in store.
N.B. This is only one example of the numerous so called "Roland's Horn". But it does link Roland firmly to the ancient pilgrimage route.
Peaceable Projects Inc. is a U.S.-based non-profit group that brings the vast resources of the wide world together with the ongoing needs of the people who live, work, and travel on the Camino de Santiago pilgrim trail network in Spain.