I think Watchtower focuses on normal, natural and healthy wholesale transferences whereas AJWRB is pointing out that cellular blood components are known to also transfer via placenta (though not on a wholesale basis).
The problem with Watchtower's premise is that it fails to apply it consistently for what it represents in relation to placental transference. For example the very 1990 Watchtower you quote from above states of "plasma" that it does not cross the placental barrier "as such" hence "plasma" is not left for each JW to accept based on personal conscience. Yet Watchtower policy allows JWs to accept transfusion of plasma products rendered from blood such as cryoprecipitate and cryo-poor plasma despite neither of these crossing the placental barrier "as such". Hence the premise Watchtower offers is no sound basis to assert its doctrinal position.
Another problem with Watchtower's premise is that it fails to apply it consistently in relation to other healthy organ transfers between mother and child. See: http://marvinshilmer.blogspot.com/2011/02/blood-transference.html