My perception is the opposite. The 3080 and 3090 have GDDR6x, doubling the actual bandwidth to the video RAM. A larger cache can certainly help improving memory access, but it won't always take the place of actual bandwidth to memory.
So the 6800, up against the 3070, doesn't have this question mark, because they both have GDDR6 - and it has a lot more memory.
So while the 6800 has a slightly higher price, I'm more willing to accept that it's genuinely better than the 3070 than I am with AMD's other two cards compared to their NVIDIA counterparts. The 3090 may cost more, but it has more memory than the 6900, and AMD didn't even try to claim the 6900 could game in 8K.
Not that I think the 3090 is that good a buy, or that 8K gaming is for anyone in his right mind.