Yes, you are right, "proprietary firmware blob on the GPU which is used to boot the Pi" is what I recall as a big Pi problem. So my original wording is correct that you need proprietary blobs to run Raspberry Pi, isn't it?
"Yes" in that the above statement about needing proprietary blobs to boot the Pis is certainly accurate & true (ongoing science experiments notwithstanding). "No" in that we somehow went from talking about "how apps can safely assume to rely on accelerated rasterization instead of software rasterization" to "how the Raspberry Pi family needs proprietary boot firmware to do anything with app output at all" and I've missed the connection between the two points.