Do I understand correctly that written instructions have a priority over videos and I'm supposed to consider perspective in this exercise right from the beginning?

That is correct - because of the nature of the course being one that evolves to account for what we've learned through providing students with feedback and identifying patterns in where they tend to run into difficulty, in combination with the fact that videos are very time consuming to update and text is much less so, there are indeed contradictions between the video and the text, and the text should be considered to be the most up-to-date.

It is something we're chipping away at fixing (in fact, we plan to drop new videos for the first section of Lesson 2, which includes the organic arrows exercise, this coming Saturday - we'll be committing to that with some public posts about it tomorrow most likely) with our overhaul of the video/demo material, but it is a slow crawl due to most of our resources being taken up providing official critique and managing the community as a whole.

The updated material set to go out does put more emphasis on explaining the application of foreshortening to the positive and negative space of the arrow, although it initially introduces drawing the arrows by setting this aside, so that the issues can be considered one at a time over the span of the whole video.