A couple of thoughts ...
Patenting a technology is not the same as a technology existing in working condition. Internally Sony’s R&D teams would have access to the 6GHz Cell processors which (certainly) have enough processing power to fully emulate the PS2; and they have probably been building a software emulator for the PS2 since before the PS3 launched. As a result there probably has existed a working PS2 emulator within Sony for quite awhile. The problem Sony has probably faced is getting this emulator to run consistently at a decent speed on the default hardware configuration of the PS3 to play enough games to make the release of the emulator worthwhile.
Now, in the process of trying to improve performance of their emulator they have probably learned a lot about how to emulate one architecture on another architecture efficiently and have probably come up with some new solutions which are patentable. Knowing how "Dangerous" patent trolls can be, and believing that they may eventually release an emulator that uses these solutions, Sony has patented them.
This (however) does not mean that Sony has gotten software emulation to run at a decent speed on the PS3's default hardware configuration.
