Humor him Nikorr and use the thread count column, iTunes encoding is primarily single threaded because it cannot distribute the load to more than one core, its encoding profile is a serial task so the encoding itself is a single core operation but itunes itself has multiple threads.
On the mac i am on ATM, the iTunes has 14 threads when open, most of these are light threads and not exceptionally active, but no program these days is only a single thread, there are always monitoring threads waiting to interrupt it. They are really small so they look like noise on the CPU usage graph when encoding because the primary encoding thread maxes out a core, but the rest of the program is still running in a dozen or so threads spread across the rest of the cores.
Tom's tends to use improper grammer and misleading statements in their reviews. When they say iTunes is single threaded what they actually mean is that it scales very poorly with an increase in the number of cores and threads available to be executed on, not that everything exists in a single program stream.