No, indeed they were not sorted. But I saw that the the .bbl file was sorted. I then found that when you add sorttype=bbl as an argument to \setuppublications (so, in your example use \setuppublications[refcommand=authoryear,sorttype=bbl]) and use \placepublications[] you will get the cited publications sorted alphabetically by author name.
I later saw that this solution was already given by Aditya Mahajan in ntg-context Digest, Vol 76, Issue 141 Date: 29 oktober 2010, Message 4, Subject: sorttype for bibtex
On top of that, this is also documented in the bib module's manual.
Except that doesn't seem to work with the bibliography being placed using \placepublications in its own chapter in the backmatter. A similar issue was reported in 2006 http://www.mail-archive.com/ntg-context@ntg.nl/msg15442.html but the solution given then involved writing \placepublications[criterium=all].