Correct. A cursory glance at the data used for that graph http://stats.wikimedia.org/archive/squid_reports shows that desktop traffic has actually increased significantly in that time period (+ 25ish% for both windows and mac). For example:
FWIW, I have first party analytics data on a top 50 online site and this was the trend we saw too. Can't remember if there were any meaningful trends in time spent though (meaning, if desktop was losing to mobile on a time spent basis).