Interesting that Arthavaveda 13,4, which appears on its face to be a hymn praising Indra as encompassing all the Devas (see esp. 46-7), in effect a vishvarupa of Indra, is apparently universally treated by Western scholars as a hymn to the Sun. Seems ideological to me?

"Pagan solar monotheism" has been a popular theoretical construct since the late 19th century. The real issue, though, it seems to me, is a defense against the recognition of polycentricity.

There is a historical narrative, too, which prefers not to acknowledge what seem like manifestations of bhakti in the Vedic era, attributing it rather to the development of "sectarianism".

Very similar Egyptian hymns (including, e.g., the acclamation of the deity as "one"—unique) used to be treated as evidence of a monotheistic "tendency", but this was recognized at a certain point as simply wrong (on which see esp. Hornung, Conceptions of God in Ancient Egypt).

When multiple different Gods can be acclaimed in this way, there are two ways we can treat it: either that these Gods are all just meant as "names" or "aspects" of some One (monotheism), or that the hymns really say what they mean (polycentric polytheism).

It seems strange to me to claim that someone who writes a hymn of extravagant praise to some God, must have intended not to praise *that* God, but some Other.

Even if the hymn in question is about the Sun, or about Brahman, this would not affect the point about how it relates to other expressions of devotion to different objects, all of which ought, I think, to be taken at face value as a baseline.

To return to the hymn itself, note how Whitney smuggles in "as" in line 2. Otherwise, we would I think read Indra here in light of the strong reference to Him at 46-7, which Whitney finds "surprising": https://en.wikisource.org/wiki/Atharva-Veda_Samhita/Book_XIII/Hymn_4

12f: "He himself [Indra] is one, single (ekavṛ́t), one only. These Gods in Him become single" expresses the logic, namely that all Gods are in *each* one, so that we will find nothing strange about similar praises being directed to any number of others.

The series of lines 29-39 is also very significant, in that it shows the constant shifting of perspective which is possible, moving a God from "center" to "periphery" and back again. This, again, expresses a logic basic to polycentric polytheism.