Wine-pthread is the only available option now and really was for the past several years. Wine was and still does supports full multi-threading. Your guess that something got simplified is wrong. What was single-threaded still is single-threaded. This for most part includes whole d3d and of course wineserver. As for some games not using more then one core - most can't do that even on windows. What you might see on windows is a "fake" multi-threading - where same thread being switched from one core to another. While on Linux threads tend to get "assigned" to a particular core for their entire life. A lot more complicated then that of course, but good enough in layman's terms.
