There are a couple of things that are being glossed over:

Hardware failures and automated fail overs. That's a thing AWS and other managed hosting solutions do. Hardware will eventually fail of course. In AWS this would be a non event. It will fail over, a replacement spins up, etc. Same with upgrades, and other stuff.

Configuration complexity. The author casually outlines a lot of fairly complex design involving all sorts of configuration tweaks, load balancing, etc. That implies skills most teams don't have. I know enough to know that I have quite a bit of reading up to do if I ever were to decide to self host postgresql. Many people would make bad assumptions about things being fine out of the box because they are not experienced postgresql DBAs.

Vacations/holidays/sick days. Databases may go down when it's not convenient to you. To mitigate that, you need to have several colleagues that are equally qualified to fix things when they go down while you are away from keyboard. If you haven't covered that risk, you are taking a bit of risk. In a normal company, at least 3-4 people would be a good minimum. If you are just measuring your own time, you are not being honest or not being as diligent as you should be. Either it's a risk you are covering at a cost or a risk you are ignoring.

With managed hosting, covering all of that is what you pay for. You are right that there are still failure modes beyond that that need covering. But an honest assessment of the time you, and your team, put in for this adds up really quickly.

Whatever the reasons you are self hosting, cost is probably a poor one.