It wasnt meant as speculation, just as an example. I should have used Angron were it makes more sense in the fluff as I think he is mostly mentioned working with daemons instead of CSM ( I think a group of bloodthirsters especially.)
As far as my speculation on what they said goes, I actually think that any and all returning primarch will have the keyword of his legion when it comes to rules, but I think at the time they made the statement keywords werent yet a 40k thing.
1. Angron has an honour guard of Bloodthirsters, which he was described as having while leading 50,000 World Eaters. So it would not be accurate to say that he is mostly mentioned as working with daemons. They are not going to bring back the Primarch of the World Eaters without the World Eaters.
2. Not mocking you, but do you honestly even remember exactly what these rumours said, and when they first came about? I'm willing to bet you don't. That's the big problem with that particular set of rumours, and why I rail against it so much: because over the last 3 years it has been distorted beyond reason from something that didn't even seem realistic to begin with.
Let's just stick to PA for now. Primarch speculation can be fun but it really doesn't have a whole lot to do with PA at this stage. If/when Valrak returns with something more than an allusion they can take their place here.