Short answer is 'bugs', possibly even in the reviewer's own software. Opus (and I would fully expect AAC-LC) preserves time alignment. Something unknown, somewhere unknown, in an unknown part of the unknown software chain caused an unknown shift, and it's not necessarily by an integer number of samples. You can't use this 'enh, whatever, good enough' approach and expect to do meaningful null analysis, even if you're using it inappropriately.

In all seriousness, every aspect of this comparison is somewhere between deeply flawed and invalid. No point dwelling on just one part.

I imagine the lossy encoding process could use a bunch of stacked filters to cut away imperceptible frequencies, but filters in the general case are implemented by summing delayed copies of the input and can smear the output in the phase domain. (There’s a branch of filters that avoid this but require computing the introduced delay and shifting the output to compensate.)

It’s a noticeable problem in audio production if e.g. a filtered kick drum goes out of phase and sucks amplitude when mixed with the original.