@Jovan ******
"I just like to really understand what's going on" Good luck with that, this is Thailand and I am pretty sure nobody knows what is going on.
To answer your question. What I was told was, I MUST use the 800K method to get the visa (year 1), To renew the visa I could do the 65K method.
However, I use the Pattaya immigration office, which may implement policies differently than your father's immigration office.
I use an agent (because I didn't have the 800K, agents can provided that for you).
Pattaya's immigration office is known to be openly crooked so it could be the case that I had to use the 800K only because the agent would make more money. Or the immigration office would make more money (bribes).
So I wouldn't say your father's situation is exactly the same.
That will be the case with most advice you get from Facebook.
Different immigration offices - different rules
Different nationality - different rules
Different "friends" - different rules.
The Thai immigration site is probably a good place to get "guidance" but in the end, its up to the immigration office which/how rules get applied.