Brainstorm the assumption

Suppose my project deadline is two weeks from now (say from 1st of June). As I know that I would have delivered the project deliverables by 1st of June, I plan a vacation from 2nd June.

I now have time from now till 1st of June, to meet the deadline of the project. I would work harder than usual, thereby increasing my productivity. Ultimately, I deliver the project on 1st of June and proceed on vacation from 2nd June.

Now think about it: Was it my planned vacation or was it the project deadline (1st June) that increased my productivity?

Clearly the latter (project deadline).

However, if the argument is concluding that the former (planned vacation) was the reason for my increase my productivity, then the assumption is that latter (project deadline) was not the reason.

That's exactly what the official explanation says.

So the assumption in this argument should be they PLANNED a vacation to occur right after a big deadline or other busy time. Because the author concludes that vacation motivated the employees. Isn't it?

No, just the opposite: They did NOT plan a vacation to occur right after a big deadline or other busy time.

Note that in assumptions, the conclusion is always correct. The conclusion is that the knowledge of vacation maximizes their productivity.

If we say that they planned a vacation to occur right after a big deadline, then we are basically saying big deadline maximizes their productivity.

This is opposite of what the conclusion is here.