This seems to be an implementation oversight.  The spec says it should be exposed. It should just be a matter of:
a) Registering the factory on OffscreenCanvas in modules_initializer.cc.
b) Exposing in interface to workers in image_bitmap_rendering_context.idl

I recall that this was originally omitted because there was no known use case for it, but then we decided it should be exposed during spec discussions.

Now, I have a use case for it: The web app receives encoded png data via websocket on a worker and we want to get the data to screen as efficiently as possible. the way to do this would be to:
1) wrap the data into a blob
2) decode using createImageBitmap
3) display to an Offscreencanvas using transferFromImageBitmap