Trying to clarify:
APC can do two things: 1) Keep the compiled php opcodes, so php execution is faster. 2) Allow the application to store values in the web server memory (kept accross requests).
ZendOptimizer only does 1.
MediaWiki only needs to be changed for 2, since 1 is done automatically by all php opcode cache.
You can't use 2 once you have several servers. An alternative for 2 is to use memcached or another cache that allows to be accessed from multiple servers.
The «APC is a "must have" for larger MediaWikis» is due to 1. In fact, wikimedia is not using APC for 2, but memcached.