No, existentialism isn’t reductionistic. Existentialists assume the reality of subjectivity, which is why they often couch their discourse in the heavy-going language of phenomenology. The phenomenologist actually brackets objective nature to more accurately describe the qualitative aspects of experience.
I combine existentialism with philosophical naturalism and cosmicism. So I like to stress both the objective and the subjective sides of our situation.
Capitalism is reductionistic or at least regressive in its unsustainability, as I’ve explained in some other articles. Capitalism encourages animal behaviour (selfishness, greed, etc) and the betrayal of our potential for more anomalous, creative conduct. Capitalism thus also encourages objectification or sociopathic heartlessness. Thus the top one percent can rationalize the crimes that lie at the root of their fortunes, by presuming the masses aren’t worth as much as the wealthy, because the masses can afford only to sleepwalk through life.
Existentialists don’t agree on everything, but they’d certainly emphasize the universality of our predicament, such as our common finitude and choice between authenticity and bad faith.