I have not seen math that supports this assertion. The table IronFighter posted seems to support the idea that Tradition opener for border growth is compatible with waiting a few score turns. Your example with Korea was a few tiles -- so why not just buy them?
Playing Shoshone makes me think I need to pick up the Tradition opener for my Liberty play -- but only after Collective Rule. I don't think Liberty Shoshone needs the Tradition Opener at all.
There is very little question that with the Shoshone, due to their UA, the Tradition opener is less useful. I was talking specifically about the Korea game. Basically if you do the math, a Trad opener grows borders 3 times in 20 turns (for simplicity I have assumed that you open the first policy on t0) while with the Liberty opener it takes 40 turns to grow 3 tiles. And the divergence gets worse after that.
For the math, 2 variables are at play here, on the one hand the extra 2 cpt from Trad opener and on the other hand the significantly faster border growth as per Ironfighter's table. Combine the 2 and you will get the result I describe above.
In the Korea game, where I was planning to plant 8 cities and build NC by t80 (missed by 4 turns), I had to micro every single tile and needed every single hammer, food and gold that I could get. I knew I would have to rush buy tiles in the expos, especially the Petra expo as I wanted the desert hills asap, so wanted to save my border buys as much as I could, otherwise I would be paying 290 gold very quickly. The Tradition opener allowed me to continuously work 2 food, 1 hammer tiles for the first 60 turns in the cap, and those tiles turned quickly into 2:2 or 3:1 tiles after improvement.
This was necessary as my plan meant that I was continuously building settlers for aproximately 40 turns with 4 pop, one settler 11 turns, I built the first one as soon as I got to 4 pop, then the collective rules settler then another 5 settlers, some took 6 turns some 5 turns, and I built one settler in my expo number 1, Busan. I was able to do this without having to spend a single piece of gold, gold that I badly needed to rush buy my archers, needed to deal with barbs and do quests. Also, I was able to NOT increase my rush bought borders counter and that allowed me to rush buy tiles cheaply in the expos, this was essential because I had to capture some of the luxes and NWs within my borders asap, as I needed all the happiness I could get (8 cities Liberty!). All of this at the cost of delaying collective rule settler by 4 turns.
Eventually, this resulted in a t209 SV with Liberty, which is not as great as Acken's 198 (mindblowing!), but it is a fantastic result for me.