You do understand eAccelerator is just an opcode caching system, aye? It compiles PHP and then caches the opcode on the server.
APC is another opcode caching system that also has data caching abilities.
If you run XenForo on a server, it will run faster with eAccelerator installed, as eAccelerator will cache pre-compiled code. APC will accomplish the same thing, without entering so much as one character in the config file. However, APC can also cache data, if you want to configure XenForo to take advantage of that additional ability.
The answer to your question is no, XenForo does not "use" eAccelerator. A XenForo site most certainly benefits from eAccelerator's cached, pre-complied PHP. If you choose to use APC for the same purpose, there is nothing to be added to the XenForo config file. That will only be necessary if you want XenForo to "use" APC's data caching abilities.
Are you beginning to see the difference between the two scripts?