Very simplified way of looking at it when for you it is something natural and easy though.Depends on position I reckon.
Is what middle forwards do always considered tackling? More like cuddle, hold, wait for help. That doesn't require a lot of technique. Cutting down a guy around the legs as he tries to beat you on the outside though. Yep - need some technique for that one. You'd want a bloke who had at least played some defence coming through the system (yet another argument for junior athletes to not over-specialise).
I'm sceptical until I see an athlete transition from another sport as an adult though. We've seen enough guys fail to transition from Rugby Union, which is at least close mechanically. There is obviously more to being good at RL than being able to run and tackle.
Gus talked about this on his podcast last year. He said it defies logic that the only people in the world who can take hit-ups and make tackles happen to be born in a 2000km stretch along the Eastern seaboard of Australia... but unless a player learns fundamental RL skills as a teenager he won't learn them.
I played against that American bloke the Knights brought out in the 90's. Greg Smith. He was pretty ordinary.
The NFL (wannabe) convert who can muscle down Payne Haas and then Patrick Carrigan in back to back tackles is probably pretty likely to get blown out by the next scoot to come his way.
I agree tackling another middle forward doesn't take a lot of 'tackle technique', I played with virtually no tackle technique and was a good middle defender. There are elements of wrestle and ball control which you can be taught and develop over time which would be tricky to shove down someone's throat.
When Sharks were here and I went to their training I was watching with a bloke who did probably 10 years with NRL WA development, out of the game a few years now, and he spotted we were practicing tackling in a different way than the NRL had taught him to teach people previously. Also watching with current first grade coach of my old club and his initial instinct was no point trying to implement that as it is just likely to confuse people/
I guess less of an issue if you were learning that from scratch but if it got implemented while still learning the old way it'd be a headache.
We know there are plenty of people around the world who can tackle and take a hit-up. The trick, as you say, is learning the skills young enough to develop through and be able to implement them at a high level. Most Union converts would probably do better if they spend a couple years playing cup and refining their skills rather than trying/expecting to be thrown straight into NRL.