So why do they exempt Ukrainians from the need to get work permits?
You answered it yourself: because these jobs are seasonal. And they are specific: simple, manual jobs mostly in agriculture and construction. Work permits are for steady jobs all year round; any jobs.
No, he only has to file a declaration, not an application.
The farmer has to declare that he can't find Polish pickers. Then his declaration is checked in the regional unemployment office if it is true. At least in theory, because in practice everyone knows that there are not many Poles willing to do seasonal work in agriculture in Poland. The government has to approve most of farmers' declarations because there would be a shortage of some 200,000 seasonal workers in agriculture. But it doesn't mean that the government is encouraging Ukrainians to work on Polish farms. The government would prefer Poles doing low paying seasonal jobs to lower unemployment or to discourage many of them from taking early retirement and working off the books.