In this program, the homeowners know. The primary mechanism of this program is that the purchaser earns interest until the taxes are paid. There is a 6-month window with multiple notifications, where the homeowner can get out of this with very little extra cost (of course, paying the actual tax would generally avoid the issue altogether).
In this case, the guy had dementia, so maybe he was not able to understand what was happening. If a private company did this to a person, the government would step in to stop it. But in this case, it is the government doing it, so there is no protection.
After the 6 months, the homeowner still has the standard foreclosure rules protecting them; the problem there is that the 3rd party involved can charge legal fees related to the filing, just like your mortgage company could do if you stopped paying the mortgage.
The sad part is that if this guy had a mortgage, the mortgage company would have made sure the taxes were paid, since a tax lien would wipe them out as well.
If the guy had been upside down on his mortgage the company wouldn’t have bought the lien. It’s a money maker.