I think btilly meant to answer this with "So at the moment of divorce there was unhappiness. But what marriage does not have periods of stress?" It isn't very clear, but the point is that something might be wrong, but it might be temporary.
> Secondly, once this person goes to therapy and ends up asking for a divorce, what makes the other person such a reliable source on whether things were going well?
A lot of decisions to get divorced aren't mutual. Many of them seem mutual because the other person decides to cooperate, but that's different from arriving at the same decision independently.
Finally, btilly said that it's an edge case. My reaction was to think that Louis CK is a comedian and that he's saying something that is the case much of the time is the case all of the time for effect. I take philosophical statements by comedians with a grain of salt.
When the person who asked for the divorce concludes that the marriage actually was fine until they screwed it up on the bad advice of the crappy therapist, that's pretty good evidence that the marriage fundamentally had nothing wrong with it.
As for going into therapy, people do that all of the time for all sorts of reasons. I can personally name people who went into therapy because of poor childhoods, work stress, seasonal affect disorder, losing a child, work problems, and so on - there is a long list of reasons that have nothing to do with the marriage that might lead someone to ask for help. Of course if the person who is supposed to be helping hurts instead, there is no limit to how much collateral damage might happen.
The common trend is that married couples start off with some reassurance rituals to let the other know that you're still madly in love. Over time the rituals get abbreviated, but remain as a sign of reassurance. However they've become habit, even if there are problems they will be kept up.
Then when the problems get to be too much for one person, the other is blindsided because, "(S)he gave me a kiss and told me (s)he loved me every morning!" OK, (s)he did. But did (s)he do that because (s)he thought you expected her to, or because (s)he meant it?
(Disclaimer. I've been married for 22 years. My perspective is that if you don't know what can go wrong, you are left with just hoping that it will go right.)
Maybe the person going to therapy was just going through one of life's blips that everyone experiences, but even assuming there is something more serious then why does it have to be marriage-related? Maybe they are suffering from depression, or PTSD, or... etc. etc. There's many valid reasons for therapy and not all of them are related to your love life.