I can't find any hard evidence for that, besides a bunch of people repeating something someone else said. I'm looking for either actual pictures of a motherboard, or well researched documentation, like from homebrewers or official Sony docs.
From what I can find, the motherboard used for the SCPH-7900X (790XX is technically more correct, since it ranges from
79001 to 79012) was separated into a CPU+RAM chip and a GPU chip (plus extra chips like the disc drive controller). There is a
picture of it's motherboard on the PSXDEV wiki, and the largest chip near the bottom-left is the CPU+RAM (
CXD2976GB), and the chip above it is the GPU (
CXD2980BGB). I can find no pictures of a motherboard with combined CPU+GPU+RAM, or any reliable docmentation of a chip like that existing. The later SCPH-900XX seems to use the
same pair of chips that the SCPH-790XX boards used, and is just a rearranged version of the SCPH-790XX motherboard.