Right. I was mistakenly picturing the channel as one conductive region, as if we were talking about depletion-mode FETs.. Still, two diodes - but my point stands, that there is always a diode there. The substrate can't be left floating, so it has to be connected somewhere, and then you have one diode each from the substrate to source and drain. It's not right to say that you can avoid the diode entirely by building it inside an IC, or that small MOSFETs like BF998 don't have one, even if it is technically incorrect to call it one diode.
In other words, regardless of how you explain it or how the MOSFET is constructed, a diode tester will always be triggered when it's connected from the substrate to either end of the channel. Always, no matter what kind of MOSFET it is.