A few helpful excerpts from different parts of the article (the rest of each paragraph is also useful):
> Users may be upset that the new processor range only features Vega 8 graphics, the same as last year’s design, however part of the silicon re-use comes in here enabling AMD to come to market in a timely manner.
> As mentioned on the previous page, one of the criticisms leveled at this new generation of processors is that we again get Vega 8 integrated graphics, rather than something RDNA based. The main reason for this is AMD’s re-use of design in order to enable a faster time-to-market with Zen 3. The previous generation Renoir design with Zen 2 and Vega 8 was built in conjunction with Cezanne to the point that the first samples of Cezanne were back from the fab only two months after Renoir was launched.
> With AMD’s recent supply issues as well, we’re of the opinion that AMD has been stockpiling these Ryzen 5000 Mobile processors in order to enable a strong Q1 and Q2 launch of the platform with stock for all OEMs.
They tried to do a "build it and they will come" but the software didn't come and Intel ate their lunch with low effort iGPUs. Advances of GPU programming slowed industry wide, especially software & dev experience / programming language wise, and the GPU world remained far from CPU programming productivity. (And you can still get 2 orders of magnitude of parallelism staying on the CPU, from SIMD, multicore and SMT).
They learned their lesson, and this time around came back with a product that can beat Intel in their own game.
Now for the Zen3 APUs, AMD is emphasizing pin for pin compatibility; possibly switching the GPU would have needed different pins; or they just wanted to make sure they got Zen 3 APUs out quickly to keep making inroads into the laptop market.
There's a leaked AMD roadmap floating around that has a Van Gogh chip with Zen2 + Navi coming out sometime this year. But that roadmap didn't show the Lucienne Zen2 + Vega chips AMD recently announced (Ryzen 5{3,5,7}00U} to ensure model numbers stay confusing.
I have a feeling that the particular market these chips are aimed at that improving the iGPU by even something like 50% is not that meaningful, it's still not going to be competitive with discrete and will nearly always be pair with one if gaming is an option on the particular laptop.
And so I suppose the rationale might be that it's just easier to stick with Vega and the current power draw for the iGPU as acceptable for the required graphics horsepower. Maybe one day we'll see an APU powerful enough to remove the need for discrete GPU in laptops, not for awhile yet though :).
It is a puzzling decision and perhaps the "wasn't ready when they taped out" explanation is the correct one. Cezanne seems to have been ready for a while now and just waiting for fab capacity - meaning it would have had to have been taped out in parallel with the RDNA2 architecture chips. So a design flaw in RDNA2 might have blown the Cezanne launch.
They could have used RDNA1 though and it still would have improved compression over GCN/Vega. Or ported over just the delta compression. I guess maybe they just wanted to port Zen3 over straight, use the memory controller they'd already proven with Renoir, and not take any risks?
It's definitely puzzling and I haven't heard an explanation I would consider 100% satisfactory.
But the Vega 8 in this is definitely not the peak that you can squeeze out of DDR4. If that was the case then we wouldn't have had things like the Vega 11 in the 3400G. Also RDNA2's "Infinity Cache" helps reduce memory bandwidth requirements, which would also be a relevant upgrade.
This was just a time-to-market strategy to reduce risk. Not an optimal engineering decision. This let them avoid trying to make a power-optimized version of RDNA2 at the same time they were trying to release any version of RDNA2.
Sure, they're going to be meomry bound some of the time, and it depends on what kind of bw/compute balance the GPU code is tuned for... but we didn't stop putting more cores and wider SIMD on chips either just because then current SW didn't fully utilize them.