The reason we still read these people at all (Thucydides, Machievelli, Clausewitz, Sun Tzu, etc) is because their work transcends "the historical and cultural milieu of its origination".
Time (and the accumulation of human opinions) is the real judge here and in the case of all of these writers, time has judged that they are, in fact, saying something eternally true about politics and fighting wars that transcends their historical circumstances. If you want to delve into their historical backgrounds, that's great -- and perhaps it will lead to new, valid intepretations. But it isn't wrong to read them as political philosophy, which is what we generally do.