The manufacturer's website says that the mount can hold a 15kg monitor. Digitec says 8kg. Which is correct? I think there is only one Arctic Z1 Basic or?
That is absolutely correct. However, the linked data sheet on the product page under Downloads effectively says 8kg. The manufacturer has probably changed the specification. Only the manufacturer can tell you what is ultimately correct.
Could Digitec comment on this matter? I think it is an important point for many people as the product is much cheaper than other mounts with a 15kg limit.