Except, of course, that the Linux desktop isn't stagnating with C and C++. That was the contention of the presenter, whose knowledge simply was years out of date.
I did my first Linux desktop GUI application in a "modern" language in 1998 -- python in that case. I did my first HTML and JavaScript desktop application in 2007. I am right now doing my first Qt Quick application -- which actually is a completely new GUI around an existing application engine. There heave been heaps of other options, like the tcl/tk others have mentioned which I personally am happy never to have to use again, but tastes differ.