I dont know if Spiernik is from the netherlands
But in the netherlands most hardware stores indeed sell aluminium conduit 25mm.
However i was looking around and i have a feeling that 26,9 mm
might atleast be a european standard for construction pipe.
Having this size conduit could have a few benefits:
Contruction pipe is available in both steel and aluminium.
It is cheaper (50%+) than the conduit you buy at the hardware store. (even if you buy aluminium)
It is also more readily available (Most hardware stores only have 4 tubes in stock and take a week to restock)
And the maximum lenght of a construction tube is 6 meters.... that leaves some room for imagination.
(Example of Some Dutch Construction Pipe site)