It's hard to fund development without releasing something, and at the same time, releasing it prematurely runs the risk of entrenching undesirable features/behaviors/etc that stifle development of something better.
Releasing it closed source allows them to point to something that justifies further fund raising and allowing them to control the direction of development while getting user feedback.
This isn't some esoteric language where if mojo vanished you'd lose your whole project and have to rewrite it into something else. It's python with some syntax sprinkled on top, so vendor lockin isn't a huge concern.
They didn't open source Swift out of the gate because they wanted to guide its development according to the features and plans they had for the language. Wanting to have a firm grasp of the language's core features and implementation is reasonable. After all, I believe Lattner was unhappy with some early directions they took with Swift, but it was too late to change anything because libraries and such had already been written. I may be fuzzy on that, but that's what I seem to recall.
Also, I am not an absolutist where free software is concerned. It doesn't make the world worse to release proprietary software. Especially if you have the stated goal of eventually open sourcing it.
An analogy would be that you don’t believe in human rights if only 10% of your business operations violate them.
Software freedoms are important. Don’t release proprietary software. Doing so is bad and makes the world worse.