You can sear however and as much as you want to. The point is that your searing isn't limited and compromised for the inside of the steak. You're doing the inside and the outside in two separate steps, and you can get the perfect cooking you want on both, instead of trying to balance things.

That's also how the reverse sear method works, but sous-vide is more reliable and low effort. You can actually leave it in the water bath for longer without worrying about over cooking. There's a reason most restaurants use the sous-vide method, they leave it in there and quickly sear to perfection when the order comes in.

There is no science to backup the fact that a "temperature gradient" is better. Medium-rare steak is objectively better than well done steak, and a "gradient" is basically part of your steak being "well-done" for the sake of the inside not being "rare".