John, your confusion in the context of the lesson is understandable, as it seems at first glance to be contrary to the text that suggests ‘not for a long time’ or  ‘not for ages’ in translating ne ..pas (passé composé) depuis longtemps. 

So how to understand why it’s correct ?

1. The translation is not direct - a direct word-for-word translation would not sound natural in English, and nor would it reflect the meaning of the French expression correctly.

2. The ‘arriving’ was an event and is finished, so passé composé is correct. 

3. This means that he is still “here”, and he arrived recently, not ‘a long time ago’.

 4. In French, this expression is regularly used in the same manner as ‘He hasn’t been here (for) long’ is in English - hence the translation given. 

I guess in English, it is not too dissimilar to: 

‘How long has he been here?’  answered with ’‘He didn’t arrive/get here long ago’ being understood to mean ‘He hasn’t been here long’.

I do think that this particular example would benefit from further explanation in the lesson because it does initially seem to be different to the others.

I am also having trouble understanding the rules around this particular example.