By that logic, no one should complain about anything, because we don't live in literal hell. But that is of course preposterous.

They're not saying there's no reason to complain. They're pointing out a very valid phenomena. For those of us on the total outside, it helps us temper bad feedback. Otherwise, you can end up with a situation like the following:

Person A from place without infrastructure X: wow, life with infrastructure X sure would be better. I wonder if we should invest in X?

Person B from place with infrastructure X: There's lots of problems and it annoys me daily.

Note that in this situation, the wrong conclusion is to say "well, then maybe infrastructure X is a bad idea". We want to avoid that line of thinking (since it's not an appropriate conclusion given the statements), so it's worth pointing out many tempering factors, such as those mentioned by the parent comment.

Parent: people often complain because it’s the worst they’ve ever had, take it with a pinch of salt.

You: well you can’t complain about anything then can you!?

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You can complain, but it’s good to have an understanding of what you’re complaining about, the parent is right. We are quick to complain about imperfection when a lot of what we have is truly quite good.

Sometimes it’s legitimate criticism, sometimes it’s being spoiled.

Any criticism that is not specific can (and probably should) be dismissed outright. Complaints and criticism should always be specific.

It's good to complain and improve things but having moved to the US I now miss even British trains!

That’s not what I meant. Only that the fact that people complain about X in both A and B does not tell much about the relative quality of X in A compared to B because the expectations and standards may be very different.