Sure, it's a weak justification. But by RAW, it's allowable, unless the DM says no (because DM beats RAW).
Nope that is not RAW.
RAW pick a domain. Choose a deity associated with it.
To fully quote the PHB (p.58):
Choose one domain related to your deity. (So deities have associated domain. Period. there is nothing to argue here.) Then we get a list of the domains followed by...
Each domain is described at the end of the class description, and each one provides an example of a god associated with it.
This means that (for example) If you pick Knowledge the gods associated with are (from the PHB under Knowledge)
Oghma, Boccob, Gilean, Aureon and Thoth. (beginning of the paragraph)
An other set of gods that can have this domain are:
Gond, Reorx, Onatar, Moradin, Hephaestus and Goibhniu. (end of the paragraph)
This mean that if you do not use appendix B of the PHB. Your god, if you want to take knowledge, is restricted to these 11 gods. You have no other choice. Of course by using appendix B, you get a whole set of gods with associated domains and portfolio. You might even get more if your DM allow the Dawnwar Pantheon! But even then you will be restricted to what each individual gods have as a domain.
So RAW, gods have one or two domains (some have three, I know...) and you get to pick one of those only. You got no other choice in this matter. This is also RAI. The player can not decides which god provides which domain. That much is clear. So unless you and your DM are cocreating a campaign world, you can not choose the domains that the gods will have. That falls entirely in the DM's prerogative. You have to get the DM's accord to change which domains a god might have. So tough luck to impose your will as RAW, you can not choose to modify the list.
The list of domains following the "choose one domain related to your deity"
does not mean that your deity possess all these domains. This is simply a list of existing domains. Period. Your god will have one or two of these domains available. Nothing more nothing less (and yes, with some of the splat books, some deities get to have three. exceptions exists you know?) But even if a deity has three domains, it does not mean that you can pick from any domain. It just means that you have more choice of domains. You get to pick one of three, instead of one of two or one of one.
I really do not understand how anybody could read these sentences and assume that all gods could grant all domains... A DM can change things. But in that case, it becomes homebrew and not RAW and certainly not RAI either.