The Playstation 2 does in fact use a 128 bit MIPS core. In fact, the PS2 has a 32 bit MIPS processor in addition. You can see for yourself from the MIPS press release:

Counting bits is not near as meaningless as counting Mhz or MIPS. Word width is a fundamental part of the computer architecture.

In response to novalis's response to me.. MIPS-III is a 64bit ISA. Furthermore, the article you refer to mentions that the registers of the CPU are 128 bits wide. That is the architecture width.