That they aren't going to dual-license 5.2 under the OGL is the next thing to meaningless. CC-BY 4.0 is quite permissive, and it's very easy to use material released under it with OGL material (ask your lawyer if you're vague on it). If you're used to the open source software world, it's roughly analogous to combining three-clause BSD (CC-BY 4.0) code with GPL 2.0 (OGL 1.0a) code.
(Combining CC-BY-SA and OGL would be likely impossible, for all that both licenses have basically the same share-alike goal. The fact that CC-BY does not have share-alike provisions is why it is pretty comfortably compatible with the OGL.)
I'm not familiar with the open source software stuff.
Looking at CC-BY 4.0, the relevant provisions seem to be section 3(a)(1) on attribution; section 3(a)(4):
If You Share Adapted Material You produce, the Adapter's License You apply must not prevent recipients of the Adapted Material from complying with this Public License.
And Section 2(a)(5)(B):
You may not offer or impose any additional or different terms or conditions on . . . the Licensed Material if doing so restricts exercise of the Licensed Rights by any recipient of the Licensed Material.
So suppose I publish something that includes X (CC-BY 4.0 licensed content), Y (OGC licensed under the OGL) and Z (my own content that is derived/adapted from X and Y), then - based on my initial reading of the above provisions, plus the OGL - I am required to clearly designate and attribute the X and not purport to impose any further restrictions on use/reproduction of it; clearly designate and attribute (as per OGL section 15) the Y and indicate that it is licensed for reproduction pursuant to the OGL; and clearly designate the Z, and likewise (as is required by the OGL) indicate that it is OGC and is licensed for reproduction pursuant to the OGL.
Is this roughly what you have in mind?
You can include stuff from a CC BY released document within an OGL document. The former doesn’t exclude the latter.
As per what I just posted, I think your "OGL document" would need to designate and attribute the CC-BY licensed material, and would need to make sure it does not purport to limit reproduction of that material by reference to the terms of the OGL.
So we could simply release the 5.2 CC-BY SRD under the OGL ourselves. If someone really wanted to use the OGL for some reason.
My initial intuition is that this is not correct, because you would not be able to comply with the following section of the OGL:
5.Representation of Authority to Contribute: If You are contributing original material as Open Game Content, You represent that Your Contributions are Your original creation and/or You have sufficient rights to grant the rights conveyed by this License.
My initial though is that being a holder of licensed rights under the CC-BY 4.0 does not grant you sufficient rights to grant the rights conveyed by the OGL in respect of the licensed material.