According to Conceptions of God in Ancient Egypt: The One and the Many* by Erik Hornung, this is a foreign idea that has been projected upon the ancient Egyptians, due to their frequent blending of gods:
[pp. 97-99, on the Egyptian ability to combine the names of deities, combine their attributes, and on mistaken notions that Egyptian religion originated with, or seemed to tend towards, monotheism:] Is the purpose of these combinations a clever priestly “equalization” of conflicting religious claims, as Bonnet, like his predecessors, assumed? Must gods be “equated” with one another until one finishes with a vague, solar-tinged pantheism? Such an interchange of attributes, which leads towards uniformity, is un-Egyptian; if anything it is Hellenistic. The Egyptians place the tensions and contradictions in the world beside one another and then live with them. Amon-Re is not the synthesis of Amun and Re but a new form that exists along with the older gods….
It is clear that syncretism does not contain any “monotheistic tendency,” but rather forms a strong counter-current to monotheism–so long as it is kept within bounds. Syncretism softens henotheism, the concentration of worship on a single god, and stops it from turning into monotheism, for ultimately syncretism means that a single god is not isolated from the others: in Amun one apprehends and worships also Re, or in Harmachis other forms of the sun god. In this way the awareness is sharpened that the divine partner of humanity is not one but many….
We shall find repeatedly that Egyptian deities do not present themselves to us with as clear and well defined a nature as that of the gods of Greece. The conception of god which we encounter here is fluid, unfinished, changeable. But we should not impute to the Egyptians confused conceptions of their gods… It is evidently unnatural for Egyptian gods to be strictly defined. Their being remains a fluid state to which we are not accustomed; it escapes every dogmatic, final definition and can always be extended or further differentiated. The combinations gods form with other gods are transitory in many respects and can be dissolved at any time. This fluidity leaves no room for monotheism, which bases itself on unambiguous definitions.
* The link is to a blog which quotes the book. The above quote comes from that blog: word and silence.