COA at Washington is $35k.
I don’t disagree with your point that many state flagships will cost $30k or less.
However, it’s worth mentioning that the vast majority of college students do not come from families earning $200k or more. Most of those students would pay significantly less at an HYPSM-type school with generous need-based aid than the list price of their in-state flagship.
I think these published thresholds are intended to build awareness of how expansive need-based aid can be at many of these schools and to motivate people to use the NPCs. I started seeing this technique back in the 90s when aid really started ramping up, though it might have been used prior to that. In any case, every time Harvard or Stanford or Princeton publicizes a threshold like this there are news reports and more people realize that one doesn’t need to be “poor” to get great aid. I’ve seen this with acquaintances who wouldn’t have dreamed of filling out an NPC at Stanford but did so based on a news report of tuition free at incomes under X.