That was exactly the first thing that came to my mind. But IIRC, Heidegger considered beings as manifestations of Being. In any case, it seems to me just like a semantic issue. As I see it, “is-ness” is always the same. That, indeed, makes God equivalent to “is-ness” or existence without substance or property. Such existence of course implies knowledge of it. And “I”, that is, consciousness or first person perspective or subjective knowledge in turn already implies “am” (that is, existence) – “I” must be in order to know, which it obviously does. Hence God's name “I am” and “his” peculiar nature of being insubstantial, propertyless, etc. which would make God appear as nothing. The most funny thing then is that we start all definitions of ourselves by “I am”. And that “I am” is observably without properties and substance too.