It's not at all BS, unless you are misconstruing this as some simpleton generalization about all digital media.
It's absolutely FACT that even the very best digital encoding will not/can not contain all the detail of an analog recording.
The best analogy that I recall for this is a fractal. That's one of those self-repeating images, that repeats infinitely. You could keep zooming in infinitely, and you'd see the pattern still repeat.
To some degree, that's an analog recording. There's practical limits on the actual recording equipment, but we'll get back to that.
If you took a digital photo of that fractal - even with the best digital camera ever - and then zoomed into it - you wouldn't have that infinite zoom capability You'd have what looked exactly like that fractal from a distance, looking at that photo - but as you zoomed in, you'd start to get the blur and pixelazation. All the detail would NOT be there. It's just not possible.
Same with converting an audio signal. Like it or not, an analog signal is a complex waveform that's infinitely smooth - and yes, that factors in the distortions and everything else preserved in the master recording. It's WHY the recording process is so important - what's captured there is locked in stone. But you can pull that analog signal up on an oscilliscope and zoom all you like- you won't ever get to a point where the waveform is pixelated. That is the beauty of analog.
When you encode that waveform, even to CD quality, you do get that pixelization - there is a decibel floor where the subtleties and details ARE lost, inherent in the recording process. The fact that SACD and competing formats exist was entirely to pursue a better quality than standard CD encoding.
Sure - if you are listening on a pair of Bluetooth Apple ear-dongles, or in a stock car, or on a bluetooth speaker in the basement - the quality of your reproduction gear won't be up to par to hear that. CD resolution was picked because it was a compromise, it was a "90% of people will NEVER be able to hear the difference!"
...but take a trip to the high-end audio section of CES some day. Some of the multi hundred-thousand dollar vendor setups in those rooms in the Venetian are set up with both analog and digital media as inputs (though the majority are analog only! $20k-$30K turntables as sources, specifically to demonstrate that high-end audio gear IS capable of reproducing all the detail in the recording!)
And the same goes for recordings - there ARE high-end recording studios, who CAN create master recordings at a level of detail that exceeds what a CD audio recording can reproduce. The fact that there's also guys with basement multi-track digital recorders making garage band and live recordings that don't contain that level of quality- that doesn't incriminate a recording format.
And it only gets worse from there...
MP3s (and similar) can be all over the board. If you couldn't hear the terrible quality of most people's Limewire-sourced MP3's back in the day, then I can't even imagine how terrible your playback gear was... All those digital formats started with already-encoded CD tracks, re-encoded and compressed them into MP3 or OGG or similar, at a bit-rate that is inherently defining the pixelation effect. You can't get around it.
And that's why it's similar to this class D argument...
Sure, it MAY be possible today to buy a high-end class D amp that has a switching power supply that makes absolutely perfect square waves, and they align so perfectly there's no overlap and no gap...
...but it's still possible to buy a lesser-than-high-end class D amp that's just intended for subwoofer use,with all the issues inherent in the old-school, price-pushing, power-above-quality class D amps that we started seeing way back when.
Not all class D is the same, same as not all digital audio is the same. It's definitely not a BS statement.