John is in conflict with Schluter and doesn't seem to understand that it is the foam that is the waterproofing on KerdiBoard. There are several ways to deal with the flange. Does the KerdiBoard go down over the flange? Did you use shims to prevent it from bowing out? You can fairly easily notch the back of it so it sits flush over the flange without the bow which will support it all along that edge...depends on how tall the tub flange is. Some are fairly tall, some are quite short (typical on CI, plastic ones are usually much taller).
If you notch it, a bead of Kerdifix on the flange would be plenty to anchor the KerdiBoard to the flange and provide a waterproof seal. The alternative is to use some Kerdiband, and seal the edge to the tub with KerdiFix.
You might want to view the videos on the Schluter site, or give their (very helpful) tech support people a call.