Pretty sure they said that just so some smartass can't come in and say that they did it before.
That said, back then it was essentially just a way for Riot to make up for a pretty stupid mistake they made.
Short version:
EU servers used to be shit, then became a reasonable alternative to NA. When european players started switching to EU they asked Riot whether there would be a way to transfer accounts, which Riot denied officially.
Once Riot did then, contrary to their original statement, offer server transfers (and in fact a once-per-account free transfer) they acknowledged their mistake and also decided to allow people eligible for such a free transfer to instead merge their account with an existing EU account.
So the only time account merging was possible was when Riot screwed up and had to find a way to make it up for the players.
I doubt it will ever become a permanent thing. Especially given how that could actually result in what would effectively be IP trading (selling accounts for merging)... or how that could be used to effectively dodge permabans in the exact way that OP intends to, which I doubt is in Riot's interest.