Plenty of companies develop desktop software for internal use. They built it, update it, and use it heavily but never sell it. I'm not sure how many of these fall into Qt's startup/small business category, though.

It seems like this license is maybe ideal for a small company that wants commercially-supported Qt to use while developing a product prior to selling it?

That seems like a fairly small niche to target, but presumably they're making this change based on actual usage data and not just conjuring it out of thin air.

As mentioned elsewhere, it's mainly about Qt for device creation. Not for regular desktop apps. For device creation you'll very likely need the commercial version to get platform support.