Digital isolator says it is bidirectional but its block diagram does not follow that statement
I would say that having one unidirectional channel in both directions across the isolation barrier counts as a bidirectional isolator, even if each individual channel is unidirectional. The claim of bidirectional communication applies to the whole family, not to any one individual IC, so that can be assumed to be talking about the ADuM1281 and ADuM1286, not the -0 and -5 variants.
Datasheets often try, especially in the headline sections, to make the most appealing claims about the product that they can manage. Just like any marketing material, you should completely ignore this information and go directly to the fine print--or at the very least, you should use the headline information for nothing more than determining if it's worth the trouble of reading the full datasheet.