It's really nothing new (not to take away from Milo's) it's a variation of something that's been around for years and since the people think that heavier is better for some reason, they are afraid to try something new light training lighter with more intensity, but I'm understanding his training correctly, he's tri-setting the same bodyparts.
Well, depends how you look at it...
Giant sets are certainly NOTHING NEW...the way I construct my workouts IS.
And as far as tri-setting the same bodypart...3 sets would make "tri-set" (so much for the Giant set theory - which considers 4 sets or more)...In one of the fitshows we did 33 consecutive sets for arms in a row...and in my training I actually did 56 (the most ever. I arranged 28 exercises and did all - 10 reps each and as soon as I finished I went - all the way back...)
It is certainly a MENTAL thing but it does become PHYSICAL...This is one of the rare times when failure to complete another rep does come from the PHYSICAL rather than mental failure...(exceptions are experienced athletes who do know how to reach physical failure as they don't set mental boundaries...You can easily test some guys by making them lift without seeing how much weight they are lifting...Instead just tell them the amount of weight - but tell them it is 20 pounds lighter than the actual weight...Not knowingly they might/will push the weight they previously didn't believe it would be possible - as they've focused on lesser amount which they know they are able to lift...Their mind is in the POSITIVE mode, they know they can do it...and they saw themselves doing it numerous times in their mind...so they just go through the same process - and complete the rep /
where mind goes - body follows/
I do think many trainees have WEAK MIND...thus they quit too early.
You've heard numerous times: where mind goes - body follows...and ONLY WEAK MINDED are looking at this training system and find the reason to complain...and NOT try it...
They would rather say it doesn't work or it is ridiculous - than actually try it and see for themselves ULTIMATE INTENSITY - they could create - if they only THINK ABOUT IT...
Start with 3 of your best lifts...whatever they might be - and try to do them ONE AFTER ANOTHER CONSECUTIVELY with less than 5 seconds rest between the two.
See how it feels...
Than imagine adding another exercise...and another...and another...and another.

Now - when you have all those exercises lined up - TRY TO IMPROVE (make more advance - using some of the Joe Weider training principles) each and every exercise on it's own...
So first exercise do in straight heavy set fashion, second do with super slow reps, third with triple drops, fourth with "double peak contraction", next with your underwear off (don't forget - I did train with Shawn once and I accepted this as a training method as well...), next: rest-pause, next some explosive power move, next (Green mamba's walk on the water with flying Tiger style)...etc, etc, etc
All joking aside -
YOU and YOUR MIND control the INTENSITY and KIND of the intensity...
You see MMA, boxer or a fighter (example) who brings the FURY in one round just to gas in the very next...Or some can squeeze 3-4 rounds before they punch themselves out...
Imagine a fighter who DOES NOT EVER STOP... EVER!
Imagine a sprinter who sprints the entire marathon....
Imagine some Mr. Olympia competitor who does his present workout ALL in NON-STOP - single giant set fashion without ever taking a break...
Whatever you imagine - you can very well make it happen...IF YOU TRY...and have a strong mindset.