The DDR5 part is possibly the least believable thing about it considering AMD won't be adopting it into any other processing lines until Zen 4, though it is possible AMD will use Rembrandt as a sort of beta test so it will go much more smoothly than it did for them with DDR4.
The only thing I don't think is right is the lack of onboard VRAM. AMD may be banking on DDR5's bandwidth and 4 memory channels to feed the GPU, as system RAM has always been a big hinderance to APU performance, especially in the budget space, but with Intel putting HBM into their Sapphire Rapids server chips, integrating HBM2 in these APUs would at least give AMD some experience, and should reduce final costs as DDR5 will be expensive, at least through the end of this year due to the chip shortage, especially for 4 modules.