That may be simply the result of those who were initially investigating the effect having cable internet connections.
But there is nothing about bufferbloat that is exclusive to "cable internet". The effect will happen anywhere that a too large buffer exists in a device with differing speed input and output links that tries too hard to never, ever, drop a TCP packet.
The effect you describe, ping latency skyrockets during upload phase of the speedtest, is exactly what happens when some device in the path of your packets and pings has an overlarge buffer that has filled. The most likely culprit is the device installed by AT&T that interfaces between your local network and their fiber network (i.e., the fiber "modem" or "router" box).