Pendant/durant.....

The linked lesson expressly states you can only use « pour » for a future event. That does not fit with one of the answers given in Céline's response that « Je suis partie pour une vingtaine de jours » is a correct option. 

As a sentence out of context, it is somewhat ambiguous as to whether the event is completed or ongoing, but it is clearly not 'future'. 

Treating it as a past completed event the choices are interchangeable between : no qualifier (as the answer seems to have been), pendant or durant. 

Can we get clarification on whether the answer here using « pour » is incorrect, or the lesson ? If both are correct, how ?

Expressing for + [duration] in French with either pendant/durant/depuis/pour (French Prepositions of Time)

Maarten, perhaps Cécile's replies to Deborah & me in the following Q&A might help:

https://french.kwiziq.com/questions/view/pour-in-future-or-the-past

In brief, "pour for future time OR intended action"

The preferred answer to the question is "partie une quinzaine de jours", but "partie pour une quinzaine de jours" is also marked correct.

Thanks Alan. Certainly useful to read, but as you noted in the discussion, the description given by Cécile is at odds with many others. As an English speaker, I would be more than happy to find more uses for « pour » than 'just the future' ! The pluperfect example seems to make sense, but if it also extends to applying to passé composé and gives a different meaning to using « pour » or « pendant/durant » for future events, it means all the explanations I have seen are significantly lacking - and they could be ! I have not found any other commentary on literally more than a dozen sites that are really consistent with it - and even the lesson here that it relates to is included in that list. I am trying to find a comprehensive French grammar reference to cover this - I don't have easy access to one presently. 

At the very least, the content of the lesson/s and the responses (labelled "correct") to questions on this topic on this site should be reviewed and reconciled for consistency.

You can see the relevant part of this grammar book online: https://tinyurl.com/y2glwq7g

The following example is given:

Elle est venue pour quelques jours. = She came for a few days.

I think you can reconcile the different lessons if you consider it as being in the future relative to the verb "venir".

Yes, Alan, I think it works for a number of verbs - venir/sortir/partir and others. I am fairly confident I have it worked out, but the lesson, and indeed the responses to date, don’t fully cover what I have concluded. Neither does the use of “future” without clarification.