Alright, I've had some more time to think about this and it's time to bring out the big guns. Beware ye of feeble heart.
This cross promotion from the app to the game can really only go two ways. One, you want people to buy the app, then go to CK2, or two, you want people to come from CK2 to the app (for the DLC). I'll give 1 a fair shake before getting to the meat of 2.
Could you consider the content a good incentive for a mobile user to check out the game? Considering one of the two events needs the newest DLC to even work this seems doubtful. If this was about making people aware of the game then this content would be unnecessary, making them interested should be enough, instead of promising a piece of music and 1/2 events. If I were to suggest a real, fair, incentive, I would say offer a free copy of an older DLC like The Old Gods. You get the mobile crowd interested with a real incentive, you don't lose out on much money-wise since it's been on sale for less, and there's no feeling of having content being taken out of reach.
Now 2, unfortunately, this move hits a certain subsection of the fan base the most - the hardcore fans. These are the ones who will see the ad since they have the rest of the DLC, they are the ones who snapped up every part of the expansion on release day since they wanted the most up to date and complete version of the game. They likely won't see this to be a good thing, and will probably feel slighted considering they just spent ~15£ and are now expected to spend ~4£ more if they want to actually have everything. This is some horse-armor tier stuff.
Suddenly we have the group of customers that will buy all your products on day one at full price no questions asked feeling ripped off, and rightfully so. This content isn't worth 4£, it's probably worth closer to 0.79£, and only that because of the music. Alienating this portion of the fan base cannot possibly be worth whatever it was hoped the app crossover would achieve.
Now, I'm fully prepared to give Paradox the benefit of the doubt here, and assume this was either not completely thought through or they genuinely didn't think it was a big deal. Actions in the past like revoking the Stellaris / Hoi 4 sign up campaign exclusivity shows they respect that players want their complete game and that they realise that certain exclusivity isn't healthy. I just want to throw my hat in the ring before this is taken further, since there really is no ceiling to the magic that is second degree price discrimination.