So basically, if I understand correctly, this is a hack that favours the miners.
And obviously, miners would be motivated to make a fork happen.
Am I correct?
What do you mean "hack"? If you are talking about the guide - it's a necessary thing to do for everyone who wants to save as much money as possible. If you are talking about the trick of creation of exclusive coin inputs - yes, it favours the miners because gives them a way to sell 1 BTC for 1000+ BTC.
Well, since I doubt anyone will be selling these newly generated coins or be lucky enough to get his transaction in only one chain,
it is logical to assume that we will see a lot of this hack (or trick if you prefer) happening by miners.
The question is: Where does that leave the average Bitcoiner who is not aware of it or lucky enough to get them coins and attempt to double them?
So as I understand it: I have 1BTC. The blockchian forks, I now have 1BTC-core + 1BTC-XT. I continue to do nothing, and this balance remains. After a time, the value of these different coins (in fiat terms) diverges. Someday, I decide to sell for fiat. If I've waited long enough, I think one of those coins is probably worthless, so I probably don't want to bother separating them. If I want to separate them, I can do so by the method in the OP.
It is worth noting that BTC-XT is not necessarily the one that gets the value (even though it has greater than 75% of the network). If the people who are buying bitcoins with fiat are more confident in btc-core, than that is where the money will go, and that is the version that will have more value (even though it's network is smaller).