This is about a blog article from 2018 from a attempt to shrink Qt to work on MCU. But since then, the Qt Company has made a new product called "Qt for MCU" which is not the same product at all. The current "Qt for MCU" is basically a different QML runtime that doesn't really use QtCore and other Qt libraries on the device. I was actually involved in that project.
But I don't know much about its current state because I've now moved to Slint [0], whose architecture is inspired from the Qt for MCU, and which can run on even lower specs, with about 200K of RAM.
[0] https://slint-ui.com