You see, I disagree with you both. This was discussed at the very beginning of the project.
This is my opinion on the matter:
It's great that they're separate parks - 1) for fans of Ferrari, or F1, or motorsports of any form might want to go the new park without going to the main park as they may not be the biggest fans of rides. By having this as a 'sector' in the new park, they could well be put off by going and therefore, PA would be alienating clientele.
2) It's not just the park being built, that huge hotel is also being constructed, and that alone will make this project feel much more substantial.
3) The areas of PA are not defined by singular product in their own rights - i.e., they are not 'themed' to an IP, (apart from Sesamo, which was to entice younger audience by creating a defined mental preset of an existing product). If Ferrari Land became part of the existing product (the PA park itself), this would become very disjointed and not 'flow' easy enough.
4) The fact it's a 'new park' is much more impressive on paper and in all press releases than saying 'a new area of XX park'.
5) We don't know what the deal with Ferrari was? For all we know, this could have been a clause of this happening, either by the park itself or by Ferrari.
6) Due to the location of this new park itself, it would be a logistical nightmare to actually 'join' it to the rest of the park with having Hotel Port Aventura right bang in the middle, then Baco on the other side of that, meaning that you'd need to 'join' it up between Baco and Tutuki, then walk all the way back, it'd take at least 5/10 minutes walking - plus, you'd see all of the 'off show' backstage areas, which are all pretty grimy up that way.
7) It's much more of a destination resort now. They've joined Disney with having '2 parks, 1 location' in Europe, this works wonders for enticing people thinking they have value for money by coming to stay at the resort, when in reality, little discount is actually offered to them.
These are all business reasonings as to why the parks are separated (in my eyes) and if I was running the park, I'd do exactly the same as what they have done and keep them separate.