The key to your question is understanding the answer you've been given.
"Farm as high as you can in under 5 minutes"
Why as high as possible in under 5 minutes? 5 minutes is an estimate or a limit value, based on a lot of math that basically predicts the amount of Grifts per hour you'll run. Remember, the guys and gals that put out these videos/statements, are the ones playing for 7-12 hours a day, and the point is to maximize their rewards for time investment. Assuming of course that you'll spend approx 10 minutes per hour, upgrading gems, in town and doing all the rum drum stuff you do outside of Grifts (including bathroom/smoke/food breaks) if you Farm 75's in 5 minutes, you'll run an average of 10-11 Grifts per hour. If you increase the difficulty to 80, but see an increase of average Grift time to 6 minutes, you'll run 10 Grifts per hour, without taking any kind of break or assuming for town or gems, lowering your average to 9 Grifts per hour. As you can see, less Grifts means less legendaries and less experience.
"But I increased the difficulty, doesn't that count for the experience and legendaries?" Not really. After you've run your solo 70, any legendary can be a Primal, meaning more legendaries, more chances.
"That didn't answer my question." Don't rush me here, the increase of difficulty doesn't mean an increase in legendary drops, it does in a way, but only after 5-7 Grift levels. That means you can run a Grift 10 and get 1 legendary, and then run a 16 and still only get 1 legendary. You could run a 25 and get 3-4, and then run a 30 and get 3. There is no predetermined drop levels, just better chances. So the faster you run, the better chances you have.
"So what if I turn down the difficulty, and run at 3-4 minutes?" That's always possible, but then you're averaging approx 18 Grifts per hour, but without the increased experience and drop chances. Also, that's a lot of keys to burn in an hour. Lowers your overall gem levels as well, as 1% does happen, and running 75's still gives you a 1% chance to 90. 70's have a 0% chance. It's all about finding that balance, and making it work for you instead of against you.