Thou dost protest too much. You are trying to justify the unjustifyible. You could not make that trap siphon even if you put a suction pump on the bottom of the drain riser. And that is how we tested traps and vents at the Chicago Cabrini Street Testing Laboratory. We did not just depend on gravity to create enough suction. And there are tens of thousands washer drains installed just like that and NONE fo them have "siphon problems", nor could they have them.