I meant "moderate" as relative to the kind of wealth that VC-funded companies can achieve, which I would label as "excessive". Obviously, 45k MRR exceeds even high wage standards in wealthy countries, but then again, even those wages hardly allow people to achieve financial independence. They're enough for them to be profitable to the government, and to maintain a decent way of life, but they still have to be mindful about their expenses, investments and financial status.
45k MRR means that you and your loved ones are safe from money being an issue in any situation, which is a liberating way to live. It doesn't allow you to own mansions, yachts and private jets, but I would argue that such wealth is disproportionate for any individual to manage.
If they can maintain this by working 4 hours a day, more power to them. That's an enviable position to be in by any definition, especially if they got there by legitimate means.
I agree that long-term sustainability might be a problem, but they seem to have the drive to keep supporting the products for as long as possible, and the ingenuity and skills to invent new ones, and hopefully the wisdom of investment, so they'll be fine. It's doubtful this would work for many others, but as they point out, this is their own journey.