Oh. I get the same error, but found an old thread.

Your link doesn't work, but I went to the archive to the mostly walking episode and fullscreen is indeed not available (yet). Previoulsy, that issue has been reported over a year ago (see this thread and this thread) and the answer there was that it was a change implemented by youtube.
Someone posted a workaround in those threads, but I do not know why that has been done for some videos and not for all of them :/