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With digital signals and ECC, the cable need only be "good enough" to get perfect data transfer through the system.

Correct. But especially if you're using long cables a cable with more "headroom" in the eye diagram will perform more reliable than one that is just at the edge of breakup.

For home use that doesn't matter usually, but I for example run events where I need the cable to work also after 10 people stepped on it and then this can become a significant thing.

Not in terms of quality, but reliability.

His link made very clear the issues of jitter and flickering.

These two statements aren't mutually exclusive. The link is looking at the analog signal through an oscilloscope. The person you replied to is pointing out that after decoding and applying error correction, you can still end up with the same digital signal output. So the eye diagram charts are useful for detecting the quality of the cable, but as long as the quality is past a certain threshold, it does not matter.

And that threshold is "baked in" to the eye mask pattern you load into the tester. If the eye stays out of the masked areas, it passes, if it goes into the masked areas it fails. Oscilloscopes capable of eye diagram testing can trigger on failure, so if it passes an eye test it'll reconstruct correctly with proper timing.

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