ActivityPub already existed when they started BlueSky. They chose to not make their protocol compatible.
Traditionally, that's what new major version numbers are for. IF (and I stress the "if" because I have no clue about protocol design) it turns out that AT has useful features, a merger of ideas of both ActivityPub and AT could lead to ActivityPub 2.0.
That would be somewhat similar to AMD's proprietary Mantle leading to Vulkan (which was originally intended to launch as OpenGL 5.0).