Doesn’t it mean that it sold more units in its first week on the market than any past Mac? How does that say nothing about sales? It’s literally about sales numbers, they are just using a relative metric instead of an absolute metric.

They announced a sales record where the metric is "sold to somebody that never ever had a mac before" combined with "in the first week of availability". The headline is worse than Cook's tweet quoted in the article.

To get this record you need to have a long time were your costumers were buying something else from you (like Phones) and have a lot available inventory in a lot of places in the first week combined with a great media coverage. 3 things that Apple has an advantage in.

As @ibero above points out it is more important for apple to tap into the vast demographic of relatively young iPhone/iPad customers - the older Mac customers are buying different machines.