OK. So if I don't care about DSD (sorry, I'm a heathen), is there anything else gained by his FIFO?
The FIFO you allude to is not a brand new type of design, it was pioneered by Wadia 2+ decades ago. Many of the best sounding CD players and DACs of the last 20 years employ a similar design, a FIFO is utilized to remove jitter at the input by briefly storing the data (in first in first out fashion) in a memory buffer and then precisely re-clocking it using ultra-spec clocks (oscillators) and very tight local power supply voltage regulation.
That simply sounds better regardless of the playback format, and prior to the last few years was something completely unimaginable as available for use on low-ticket tech like the Raspberry Pi. Previous versions of this offered by Ian had the FIFO buffer, clocks, and isolator on 3 separate boards.
This latest group buy will offer all 3 of those functions combined onto one HAT, this new incarnation is called FifoPi, pictured below with the DAC HAT on top, and an optional hardware controller board too for those that want to use the DAC's built-in ESS hardware volume control: