I don't think that's true. So first of all this whole article is essentially about the "filters" that are used to encode the data more efficiently based on the

I don't think that's true. So first of all this whole article is essentially about the "filters" that are used to encode the data more efficiently based on the surrounding data, before it's passed to deflate. Basically compute some kind of a delta from the pixel compared to the pixel up of it, left of it, or a combination of the two, with the deltas generally compressing better than the raw data would have. That's already a big difference to compressing each row separately. Second, the deflate compression does not happen individually for each row. It happens individually for each IDAT block, but those blocks can be of fairly arbitrary size. Using a separate IDAT block for each row would seem very odd. (Can you even use filters in that case, or does the state reset with each new block?)