If KDE binaries were huge, that would be a consequence of KDE design choices, not of being in C++. Most likely, though, the definition of "huge" has anyway since grown past the size of KDE programs, and Gnome sizes probably have caught up to KDE's.
I have not tried KDE in this decade, or the last. The article makes me curious. But Qubes is an Xfce desktop, and I am stuck there for the time being. It has been OK. Qubes desktop can't use a GPU anyway.
If Qt goes proprietary, it will fork without drama.
I have used both KDE and GNOME during their 1.x and 2.x days, and even the beginning of 3.x days and KDE was never outrageously bloated compared to GNOME. The difference IME was always 10-15% more RAM use. And this was understandable since KDE was a much more full featured desktop with far more knobs to tweak and many more inbuilt applications compared to GNOME.
I don't know what the situation is these days though but anecdotally it seems GNOME has now become the "bloated" desktop, thanks to Javascript in the shell.
On a fresh reboot and login to a KDE desktop, the total memory use (minus disk cache) on my system is just a hair over 1GB. This is on a ~10 year old Phenom II with 16GB RAM, and KDE is smooth as butter. It runs great on my Thinkpad X220i, too.
Firefox or Chrome will absolutely dwarf your DE in memory usage, so I don't think it's really a major differentiator between KDE and Gnome. If your PC has less than 2GB of memory, you'll run into other issues first.
Only the Qt thing is current, but memory is long on these things and a lot of people formed their opinions a decade ago. First impressions and whatnot.