I can't believe I'm about to explain to people something about fox when I don't play the character...
OK, This is a term used in other fighting games but for some reason doesn't make it to smash, and it's called option select. Option selecting is when you do one thing to cover multiple options. In SF3 Third Strike, there's a very simple option select while your opponent is pressuring you, and all you have to do is hold down back and press light punch and light kick. If they keep hitting you, you'll block, but if they try to mix in a grab, you'll throw break, putting the both of you back to neutral. You can cover multiple options without having to necessarily read your opponent.
Going back to smash, this is exactly what happens in Fox vs Fox/Falco matches. They don't necessarily read when the shine is coming, but there are certain situations where there's no real reason to not press L or R in anticipation for the tech. Any fox or falco that lands a dair is either going to shine or grab. Grab is a combo to begin with anyway so unless your opponent is missing it consistently, there's no reason to do anything about it, but if they try to shine, you'll be pressing L or R anyway and tech it. I personally also hold my control stick away from them to DI the up throw or get further away with my tech roll (stage positioning of course may change my approach).
When you see good players with good DI, it's generally not 100% reaction or 100% prediction. Sometimes you're in situations where there's nothing better to do with the control stick than to hold up because you're in the middle of an attack or whatever. I know personally in Peach ditto's whenever I down smash, I press down on the c stick and hold up on the control stick every time. I almost never ever ever eat more than one hit of the d smash, unless I'm at zero or I foolishly mashed c-stick down and I end up c-sticking my DI into another hit.
As for when I play Fox or Falco against Fox, it's not uncommon for me to personally tech about half the shines thrown out during the game.
As for the specific argument of whether it's a combo or not, I'll say that as a peach player, I'll often times get 2 or 3 d smashes in a row vs space animals. Now it's a combo in that once they missed the tech they can't do anything about it. But I mean really, if you miss the tech you only have yourself to blame for eating another 30%.
Good luck playing you're god-like character with a one frame move.
PS: Just because MarsFool! doesn't have a purple name doesn't mean you shouldn't listen to him.