philosophers theretofore were treating human beings like objects rather than subjects in searching to understand them.

When Nietzsche and Heidegger critique metaphysics it is the belief in the metaphysical "Subject" that they are ultimately getting at.

The question of the "who" answers itself in terms of the "I" itself, the 'subject', the 'Self'. The "who" is what maintains itself as something identical throughout changes in its Experiences and ways of behaviour, and which relates itself to this changing multiplicity in so doing. Ontologically we understand it as something which is in each case already constantly present-at-hand, both in and for a closed realm, and which lies at the basis, in a very special sense, as the subjectum.

Here Heidegger is giving the conventional view of the metaphysical subject as something believed "present-at-hand" and "identical throughout changes in its Experiences". Put in simple terms what he is saying is the subject is being thought of as an object.

There are still harmless self-observers who believe that there are "immediate certainties"; for instance, "I think," or as the superstition of Schopenhauer puts it, "I will"; as though cognition here got hold of its object purely and simply as "the thing in itself," without any falsification taking place either on the part of the subject or the object. I would repeat it, however, a hundred times, that "immediate certainty," as well as "absolute knowledge" and the "thing in itself," involve a CONTRADICTIO IN ADJECTO; we really ought to free ourselves from the misleading significance of words!

Nietzsche does a similar thing with his perspectivism here, critique the belief in a "Subject". Really, Nietzsche's becoming is an historical dynamic mode of being that with his perspectivism annihilates any belief one might have had in a static "Subject" - "immediate certainties", "I will", "absolute knowledge", these make no sense whatsoever from the perspective of Nietzsche's becoming.

something he thinks Nietzsche and Sartre fail to do

That is weak for Heidegger to say considering the Da-sein of Being and Time is contextually understood subjectivity without reference to historical time-frame. It was only after Being and Time and his dalliance with the Nazis that Heidegger understood his ahistorical contextual mistake.

Around the time Heidegger engaged Nietzsche is when Hediegger began to put things in to historical context with the use of Nietzsche's concept of nihilism.