Me no comprende! Games are written in C++, PS3 and XB360 alike. It's not like PS3 has its own language and they are running C++ code through a JIT compiler. Optimisations are possible and even low-level code etc. Maybe they aren't writing effective SPU code. But to say it's not built on an optimised engine, it's built on C++, makes no sense. Unless Sucker Punch consider anything not written in assembler to be multiplatform
I think the whole multiplatform thing is a comment stretched and taken out of context. You know how people are. If they hear a developer say "well, the interesting thing about inFamous is that we didn't optimize specifically for PS3, so the engine could very well work on nearly any platofrm" they take that to mean "OMG MUTLIPLATFORM!!! YES!!!".
The reality is, the game was created like any other game, however, the engine doesn't make very specific use of PS3 hardware like other titles might, therefore, it would technically be possible to move it to other platforms without significant changes to the engine.
I think the reason they made the game this way has a lot to do with their size, and with their methodology. They do a lot of prototyping, and I think with that sort of development structure, you need an engine that is primarly made at the assembly level (I think I'm saying this right). This way, they can see changes, make them easily, and don't necessarily have to worry about reoptimizing constantly for changes in the visual engine, physics, animation, etc.
Sure, they would still have to optimize, but they can prototype and polish much easier with general code, however, the end result may not be as "pretty" as a game that takes specific advantage of the hardware available to it.
....hopefully some of that incoherent rant made sense?