That's what I thought as well, that it's not needed because the 4-screen bit already indicates the presence of the extra VRAM. I also noticed however that when the battery bit is set, the correct amount of PRG-NVRAM (usually 8 KiB) must be specified separately in the NES 2.0 header, otherwise the header is considered invalid (battery with null bytes of RAM to be backed), so I was considering whether 4-Screen Mirroring is similar.
And for Zhongguo Daheng, which uses both the two CIRAM pages and four additional VRAM pages on the cartridge for a total of six simultaneous nametables,
Quietust recommended to me that I should use CHR-RAM for that, but
then that extra VRAM certainly would need to be included in the CHR-RAM field, otherwise Nintendulator would mask that memory off. Of course, that advice could also have been a recommendation for that edge case of a game using a total of six nametables together with the way Nintendulator implements CHR-RAM. Nevertheless, as NES 2.0 was defined for such edge cases, at some point the
NES 2.0 wiki page should resolve this matter explicitly.