Actually I would interpret it that way, that your interval workout was deemed aerobic as a whole and that you could increase intensity, if you want. Zonesense is not about HR measurement, Suunto says, but about Heart measurement, measuring the stress of the heart. And I think it makes sense, seeing that after your short intervals Zonesene tracks you moving into higher aerobic states.
This is by far the easiest to digest explanation of the behavior I have seen. This makes perfect sense.