That seems a common refrain as if San Francisco invented homelessness and now exports it to the world. But seriously, what percentage of voters in Seattle are actually from SF? Secondly, why do you suppose that the people who strongly support San Francisco policies are the ones who are leaving San Francisco?

No.

I think it's in some way related to the explosion in cost of living, importation of tech workers getting paid ludicrous amounts of money, and, despite record-setting construction rates, inability to match housing supply with demand.

Also, consider that the housing climate in Seattle is highly different from SF. There is no proposition 9. There is no rent control. There is no 'lovely California weather attracting rough sleepers' and no mythical 'bus-fulls of homeless people being shipped in from other states.' And, yet, homelessness is skyrocketing.

Maybe rent control and prop 9 aren't solely to blame for out-of-control homelessness? Maybe people like me are the problem?

That's interesting and fits with SF -> Portland migration as well, but were policy preferences in Seattle all that different from SF before the last few years?