I dunno hey my best mate is a chef and thats the way he was taught by one of SA's top chefs.
Not to rain on your mate's parade or anything, but he's plain wrong. I also attended culinary school and still study culinary science and I can assure you that there is absolutely no sealing happening. You can test it yourself with two steaks - weigh them before and after. One sealed, the other not. The sealed steak will weigh less because it loses moisture during cooking - when you see "smoke" from the steak that is actually liquid from the steak turning to steam - it's losing moisture. But yes, you do have to brown (what used to be called sealing) the steak first for the maillard reaction - go look it up - it's very interesting...
I have cooked a good steak slowly and it wasn't good at all, it came out all dry in the center - I can understand if you're making a curry or stew but I have never heard of someone cooking a good steak slowly or turning it often - goto a steak house and watch what they do.
I'm not suggesting slow cooking a steak - I'm suggesting a gentler heat (as opposed to a smoking hot pan) for very thick cuts to prevent drying the outside of the steak and inducing the maillard reaction and heat to the centre.
BTW, many steak houses absolutely abuse steaks - if you want to cook it perfectly, use a technique called sous vide for a few hours at 60C - this is the temperature at which the protein bonds in meat denature (which is what cooking really is) and then flame-grill the steak at the end...