How we can go to the pari-nirvana if not by being a bodhisattva?
What appears as a contradiction is the difference between Theravada and Mahayana paths. I cannot speak for the Theravada path since my study is Mahayana, but in our tradition, bodhicitta is necessary for enlightenment, eg: from the Diamond Sutra:

One who wants to give rise to the highest, most fulfilled, and
awakened mind must create this resolved attitude of mind: I must help
to lead all beings to the shore of awakening.

I have been taught that this is not a contradiction, but, rather, that Buddha gave many teachings, all tailored to the student. And that yes, it is possible to achieve nirvana, and become permanently liberated from samsara, the cycle of suffering, without bodhicitta. But that is not the path I am on. And whether or not it is possible to 'lead all lead all beings to the shore of awakening' is not the point. The point is having the intention to do so, and also, of including those you do not like.