I disagree, and think your points below (hackathons, grants, eco funds) illustrate that projects are actively trying to engage developers — not just relying on developer interest from holding tokens.
It is also worth noting how much effort & funding projects put in to bounties, documentation (!!), and community engagement online and IRL. Some even hire a developer community lead.
Good projects know how important developer engagement is — but agree that it is hard to know (yet) what approaches have been successful.