I expect there's gonna be a huge backlash once it becomes clear to Ukrainians that they were being used all along.

Well, you answered yourself, it's so fucked that nobody dared to do a census for 23 years. Though i would say that if the numbers are from the official sources they can be pretty accurate because socialist and postsocialist countries count their citizens rather accurately even without census, censuses are done to get more indepth statistics. Though in case of Ukraine immense emmigration might be the source of much of that graph valley as millions of young men currently work in EU countries and can't go back if they don't want to be drafted, Poland and Baltics are full of them for example. Also i expect Ukrainian government to manipulate the numbers to turn a few million of emmigrating or nonexisting (since as you said, it's going for a long time and it's enough to slip a lot of neverborn here and there) people into "killed by Russia" hologramodomor style.