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.
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).