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In "A Thousand Sons", Magnus has a conversation with Ahriman after Ahriman has "drank" up knowledge by staring into Magnus' essence. Ahriman mentions, quote "but you had already peered into its depths (the warp) and seen them".
It seems clear that them refers to the Chaos Gods, based on Magnus' reaction. Is this a reasonable assumption?

Magnus knew about the dameons of Chaos, he just didn't give a gak.

Apparently many of the Primarchs knew about them. It's just that, unlike the Emperor, they didn't know enough to understand the true power they possessed. As far as they were concerned, Daemons were basically just xenos existing on another plane.

Thanks for the answer. But let me define my question a little bit better. I was literal when I asked about the Chaos Gods.
Based on the information we have, Magnus was supposedly a psyker comparable to the Emperor since he was the incarnation of that aspect of the Emperor. There is hinting in the HH series that the Emperor bargained with the Chaos Gods themselves for the Primarch project (could be a lie or the truth). Hence, both the Emperor and Magnus could have encountered the Chaos Gods in the warp.
My question comes from the fact that, as I understand the fluff, the Chaos Gods do not have a singular representation. They never appear "in the flesh" and apparently they are always content to have their pawns do things for them.
I understand that it is possible that the encounter was with Greater Daemons but the emphasis that Ahriman gives to the word implies dread which would be appropriate for the deity itself (based on the Thousand Sons being so full of themselves concerning the warp).

Keep in mind that Ahriman and Magnus are having a spiritual conversation, and those are frequently couched in metaphor. He was likely just sensing them; sensing the power, the intent, the intelligence. But yes, I think he was referring to the the four major Chaos powers.

The is a part in Betrayer where Magnus and Logar are talking. Magnus refuses to call them "Gods" because words have power and all that. However he does know them as vast intellegenves in the warp.

Gods have physical appearances. Tzeentch looks like this.

That is commonly how he chooses to appear, yes.

That is not necessarily what Tzeentch actually looks like.