You are absolutely correct. When I open the valve and turn on the anchor wash pump then water flows freely into the toilet. The line retains water when I turn the anchor wash off, and the toilet can be used, although I suspect it will lose prime over a short period.
However as soon as the anchor wash is turned on again the toilet starts to fill immediately. This seems like a poor situation really.
I guess the saltwater flush mode really needs to be treated as a degraded mode of operation rather than an alternative mode of operation you could choose when on an ocean crossing for example.
Anyway, I now understand the operation. Thanks Mohammad.