I actually thought the opposite, and I don't think it's surprising. The admissions process for the 2+2 program is on a relatively much, much smaller scale than regular HBS admissions.
In the rejection letter, HBS officially stated that they had just over 600 applicants for 2+2. Contrast that to their regular MBA admissions process, in which HBS gives interviews to approx. 850 applicants from Round 1 alone. In that scenario, I could understand sending interview invitations on a rolling basis given the sheer volume of applications. In the case of 2+2, it makes sense that even up to 200-ish interviews (from a mere pool of 600+) were sent out at once. This latter situation doesn't really require a rolling interview process.
Furthermore, since HBS hasn't yet entirely defined the 2+2 identity, they had to decide on all the interviewees at one time in order to have a clearer vision of how their prospective inaugural 2+2 class is shaping up. I feel that in their regular MBA admissions process, they can conduct interviews on a rolling basis because they already have general criteria in mind when reading through applications. This is really not at all the case for 2+2, and instead they needed to read through everyone's applications and evaluate them collectively before deciding whom to interview.