Memory bandwidth is but one aspect of GPU performance, and also memory bandwidth is substantially upgraded in this APU. It has support for LPDDR4X-4267 68.2 GB/s, up from DDR4 3200's 51.2 GB/s.
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.
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.