This is FUD
You need only look at the architectures of both systems to see that they're comparing apples to oranges. They claim the Dreamcast is better because it has more VRAM and that this leads to better, more realistic environments. Their numbers are correct. But what they fail to realize is that the PS2 doesn't use VRAM for storing textures. It in fact, has an amazingly high-bandwidth bus that streams textures at 48GB/s directly from main RAM instead of having to store these textures in VRAM. The PS2 has a philosophy of low cache, high bandwidth, whereas the Dreamcast and the PC have a philosophy of big cache, low bandwidth. On the PC and Dreamcast, we rely on very large amounts of RAM to store textures because the bandwidth of the busses in the system is just so crappy. The PS2 decided to use only the very minimum amount of memory and connect the various subsystems with every so high-bandwidth interconnects.
What does this all mean? Well, it is very different to program for the PS2 than it is for the PC and Dreamcast. Programmers have been used to just throwing textures into VRAM and hoping for the best, whereas with the PS2 they will have to stream textures during every frame (but there is no problem with this because at 48GB/s, the PS2 could effectively stream >800MB of textures per second and achieve a framerate of 60fps -> that is just over 800MB per frame).
The Dreamcast is like a PC in its approach and since it only has 8MB of VRAM, one will only be able to have <8MB of textures unless some sort of texture compression is utilized.
Just to sum this up, the Dreamcast and PC use large amounts of memory with slow interconnections to other parts of the system. The PS2 uses small amounts of memory with very fast interconnections to other parts of the system. So you can't just look at one dimension of the numbers (in this case...the size of the memory) and infer anything about why this makes one system better over the other.
EDIT
I also noticed that the article pointed out that the PS2 cannot pump 75 million polygons per second because it only has 4MB of VRAM. This is pure bs. The PS2 doesn't even use the VRAM for storing polygon data. The VRAM is used solely for texture storage (one frame may include more than 4MB of textures by storing different textures at different times when they are needed to render a part of the frame), colour lookup tables and the frame buffer. The polygon data is streamed right off the bus and is therefore limited to the bandwidth of the PS2 and not the VRAM.
-GL