Maybe they mean aesthetically? KDE basically makes a clone of their Breeze theme for GTK to try to make all apps have a consistent look on KDE. I believe the reverse is true with an Adwaita theme for KDE/Qt apps, but don't quote me on that.
You can find versions for both "toolkits" even in custom themes. For the end user it's mostly a non issue. Even with different backends it looks more consistent than Windows.
That's true, but the Iced theme that they're showing in the images looks like it would be very difficult to recreate in GTK and Qt. That's not mentioning the increased amount of time and effort in maintaining those themes. Perhaps this is an area that the community will contribute to though.
The same is true with an Adwaita theme for KDE/Qt Apps. I used to do this on both, Plasma and GNOME.
These days I style my Qt/KDE apps all Breeze on GNOME and vice versa when I am on KDE, because unfortunately icon sets just don’t match up and I've run into 'invisible buttons' issues way to often.
As long as both are in dark/light mode I find this way less annoying than having to hope for a good enough tooltip on mouse over (or just tap and see on mobile).
