It's not a good idea or a bad idea, that blog post is designed to get you to think about it.
It's the same way Pete wrote a lot about relocating to an LCOL location. It's not the best option for everyone, but it's absolutely worth thinking about.
The whole point is not to be passive in your decisions. If you live in an HCOL location, you should be constantly assessing if the trade offs are still worth it. If your house that you bought for a reasonable price is now worth over a million, you should assess if the house is worth trying up that equity.
The point isn't that you should sell a house when it's worth 1M, the point is that your plans should evolve and adapt to changing realities.
So if someone is living in a certain area for work and their house had appreciated enormously in value, and they don't like their job, then it might be perfect to sell the house, relocate to a place with cheaper housing, and retire.
That may be an awesome option, or it may be a TERRIBLE option. It depends on what the benefits of staying in that house are vs leaving.
If they have 3 kids, one special needs, enormous local family support and specialist medical care, and their house is in a specific location that has actually appreciated less than other houses in their region, and downsizing isn't an option because their home is actually a townhouse.
Then no, cashing out for them would make absolutely no sense. It would be a bad trade.
Why do you live where you live? Would retiring change where you would want to live? Is downsizing a desirable option?
These are questions everyone should be asking themselves at some point. The 1M house example from MMM is just a thought exercise to get you thinking more and making decisions in a less automatic way based on past decision-making parameters.