If you want to question what is appropriate for countries to do, you still need some broader framework than the laws of the country. We could pass a law like the one in Logan's Run that requires people to submit to being killed by the government when they are ordered to die, an we would still need some basis from which we could argue this is wrong.
It is not safe to simply allow governments to do whatever they want. We started out that way, and quite often it did not go well. So the argument that whatever laws exist make whatever they command appropriate because it is legally mandated should not be taken as disproof of this concept.
The argument that 'taxation is theft' usually proceeds from the premise that force used by a vastly superior power against an individual is never warranted, and the observation that most of us pay our taxes out of fear of governmental force.
The first of these is probably what the author means to have as his 'a priori' truth. It is fairly easy to argue that most children respond positively to this idea at some age, and that we would consider those who never do to be psychopaths or to be ideologically brainwashed, and not to be representative of normal human standards. So you could argue that this is a basic notion of justice that we all would share naturally if we were not talked out of it. That is one often used, if not totally appropriate, meaning for 'a priori' when discussing ethical questions.
That obviously does not make the deduction such a truth, only the premise. To get the deduction you also need the 'observation'. And that is not obvious.
I don't feel that I pay my taxes out of fear. I appreciate being a part of my country and its economy. So this argument falls flat. The fact that some people want to cheat at a game and need to be punished does not make the rules of the game into a threat. Especially if there are ways in which one can avoid the issue. Most taxation codes, modern ones anyway, do not force people with very little money to pay taxes on necessary goods without offering them money that would more than offset the level of taxation. So there is a way to not play this game -- be poor.
If you have taken the right to amass property as another basic principle, those then conflict. But then it is the tension between these two, and not the nature of taxation, that is the problem. I am sure the author would consider this 'a priori' in the same way, and I don't. There have been levels of civilization where property was not protected or regarded, and one simply had what one could hold onto.
If property itself is a convention afforded by our social structure, and not an inherent right, then this characterization of taxes as a way to earn the right to belong to an economy, rather than as a threat against all in a region by a bullying overlord, is just as logical.