What does this overlapping ADC symbol in a block diagram mean?
I read this as multiple ADCs being available and not as a multiplexed single ADC.
The datasheet of the TSL2584TSV supports this:

Two integrating ADCs convert the photodiode currents into a digital
output that represents the irradiance measured on each channel.

In my opinion a multiplexer is a different part in the signal chain and should be represented by an own block in front of the ADC. But that is only my opinion.
Not sure where you got the first image from, so I can't really comment on that one.