That would be completely wrong. See the Fedora Miracle Spin, for example. The project (miracle-wm) is still a work in progress, and yet Fedora already officially offers a Spin for it. What you’re describing would only be true if Fedora was switching to COSMIC as the official desktop for Fedora.
There would be no reason to delay anything regardless of it being beta or not. Fedora is always doing rolling release updates of software that lends well to that paradigm, and COSMIC is one of those. It’s not like GNOME or KDE where you have to carefully and meticulously manage 100 system libraries used by the whole ecosystem. System dependencies in the COSMIC ecosystem are virtually non-existent. If they really wanted to, they could just wait a few weeks to release the spin. But I don’t think there’s any reason to.
Alpha 5 is actually releasing next week. I’d also disagree with “very alpha” as most of the beta milestone is finished now. Regardless, I don’t understand where the disconnect is. A spin can release at any time. Doesn’t matter when COSMIC Epoch 1 releases.