Bell of lost souls will write anything, just to get clicks for advertisements. Nothing you read there can be considered reputable in any capacity.
There will be more duel faction box sets for sure, as they are great for GW, and the consumer, but you don't need bell of lost souls to tell you that.
They also wouldn't ever put a primarch in a duel army box, like shadowspear or tooth and claw. They'd be seperate releases to diversify sales. Case in point, shadowspear and Abbadon.
There could be a tie-in with a simultaneous primarch release, but I am very skeptical we'll see more loyalist primarchs.