Of couse games do not care about UI toolkits, but the excuse against linux was always Gnome-KDE or Gtk-Qt-Motif-Athena-whatever, even through games do not care about these things (see also your grandparent post).
In Linux, there is glx[1]+OpenGL and SDL. The problem (quality of OpenGL implementations) is not unique to Linux.
Yes I remember now that the Valve SteamOS devs recommended to use SDL2 instead of going directly on the OS APIs, since it takes care about a lot of small problems and differences. So basically SDL would take the role of DirectX.
In Linux, there is glx[1]+OpenGL and SDL. The problem (quality of OpenGL implementations) is not unique to Linux.
[1] - yes, soon to be EGL.