What does this mean, for devs?

It means that retaining control of the design is best done by remaining your own primary customer.

As soon as you are reliant upon a third party, to finance your development, you cease to be the primary customer, even if you are contributing much more in effort than someone else is in money. Money is greater leverage than effort, as it is more liquid and fungible, so it is easy to hold adding or removing it, all or in part, over the heads of the development team.

Since the person contributing this marginal capital has more effective control, they are the true owners of the product, even if they currently choose to leave you to yourselves.