8MB was pure marketing talk. The specification of the RAM Expansion Control register only allowed for two banks of 512K each. There are still a couple of bits undefined, so more might be possible, but for the final state of the machine it was 1MB only. (Docs backing this up are available at https://github.com/MEGA65/c65-specifications ) 8M or maybe just 4M might have been included at a later state, as the register still has some bits unused.
Also there is no completed Basic 10. The most current ROM known so far (dated 911001) still contains several ?NOT IMPLEMENTED ERRORs.