>>43351405I'll test it right now out of curiosity but either way the odds of everything coming together for the glitch to happen are very low. You'd have to be on an entirely different route, decide you want a tentacool, only have decided that after encountering several of them on your way toward Cinnabar in the first place, and then choose to search for one on the coast without taking a single step to the right.
And all that is just to see normal Pokémon from other routes, say "wtf is oddish doing here" and probably never understand exactly what made it happen. Bringing MissingNo. into the equation requires an additional step that serves literally zero practical use, making the odds of finding it near zero. Of course the game sold tens of millions of copies, so those odds paid out, but no one could ever expect it to be found in testing.
Plus like I said it wasn't in RG, only in RB. So it wasn't just a testing group that failed to find it, but a testing group that was mainly focused on translation errors. The more factors you consider in this equation the worse the odds become.