Are we certain the stem angle is constant?

Nope definitely not certain. The drawing states 1:1 scale with multiple stem lengths at a fixed angle, but it could be an incomplete/draft drawing.

austtam: I might be wrong but I think stem angle is going to change depending on stem length to fit that 30mm rise.
So maybe 90mm stem could have 16° stem angle, but maybe 120mm stem will have just guessing 12° stem angle to fit in that 30mm rise dimension.
That would make more sense to me than on every stem length to have different rise, just have one (30mm) rise dimension and then alter stem angles, but that is just my speculation.

That would definitely make more sense. the "30mm stack" terminology is also ambiguous. That could technically point to the vertical height of the steerer clamp of the stem as well.
One piece riser bars by way of stem angle seems like an overcomplicated way to execute. On the product side, the stem length becomes totally different. On the production side, you have completely different shapes for every stem length. I also don't think they look better than the avro-rise shape.