The "catch" with the IBA plots is that they have very, very strict design rules, with committees and boards and such – every draft has to be approved. Because of that, the houses end up being a bit more expensive (for example, 2 floors are mandatory). Allocation is done based on the time of submission or by lottery.
At least for the plots still available now, there will be no more lottery procedure. Even for the plots already allocated, the rush couldn't have been that great, otherwise those who were rejected would have immediately gone for a free one.
The "catch" indeed initially reads as if you have to firmly pick one of the offered houses. But ultimately, it’s completely fine. The house types simply don’t represent any great extravagances. Overall, it’s just the modern standard that is built in 95% of cases. And you have to orient yourself accordingly if you don't take one of the model houses (then without committee approval).
I rather read that the houses being 2-story means "fortunately allowed to be 2-story." Most would fight over that under old development plans and instead have to deal with knee wall heights.
The extra cost for a full floor is very low compared to the gained space.
Overall, you have a fairly relaxed situation up there for an actually very sought-after big city.