He means you can't tune a non turbo car with out 'forced induction' ie putting a turbo on it. He's wrong of course.
It feels great and is quick off the mark, but doesn't have the 'kick' of my 1.9 tdi Fabia at, say, 60.
It never will, cos of how the engine is designed. It's rather clever. If you want a lot of power from your engine, you can get that by having it rev higher. If it revs higher, it needs to shift a lot of air, which means the air has to flow fast. Problem is that the air is sucked in not as a continuous stream but in gulps as each cylinder sucks air in. So if you want lots of air going in fast you can have the exhaust valve stay open whilst the intake valve is also open, so the exhaust gas leaving at high speed helps to suck some fresh air in quickly. However this causes trouble at sensible speeds because the air's not moving fast enough to make it work.
So Honda invented variable valve timing - so you have normal car engine at normal speeds, but as you approach where the red line would be it switches to racing car valve timing and you can rev much higher - which means more power.
That's why they need their nuts thrashed off, it's how you switch from normal car mode to racing car mode I quite fancy driving one, I must say, it sounds like a laugh
Diesel otoh is completely different. You get quite a lot more force per bang, because diesel has more energy in it but also because its not very volatile so you can compress the air a lot more, and this makes it more efficient. However the high pressures mean you can't do this as many times per second without the thing destroying itself.
A diesel only compresses air on the compression stroke, so you can put as much air in there as you like, and squirt as much fuel in as you like. As long as the engine can handle the pressure. So diesel remaps just force more air and fuel in and hope it doesn't break something Generally it won't, because they are built strong.