There is one sense in which searing meat really does seal it.
For a long time it was genuinely believed that searing meat in some way "sealed" it. As other answers have already shown, this is nonsense.
Indeed it's quite easy to prove:
- Sear a piece of meat.
- Roast the piece of meat.
- Observe whether the meat inflates or perhaps bursts.
If you put a sealed object that contains a lot of water, fat or air in it into a hot oven, then it's going to get larger as the water (and/or fat and/or air) expands and perhaps burst. We prick sausages precisely so that they aren't sealed.
By the same token, if there's more liquid in the pan after cooking than could come just from the surface then clearly that liquid wasn't "sealed in", was it?
However, if you refer to searing meat before roasting it as "sealing the meat" you aren't being incorrect. And if someone says they consider it incorrect we could ask them if that means they are studying astrology; consider literally means "examine the stars" but we use it to mean "think about" whether or not we believe in astrology and certainly not restricted to cases where we actually draw up a horoscope. By the same token, "sealing the meat" means "searing the meat so the maillard reaction improves the flavour and gives a more pleasant colour" even as used by people who know its etymological origin in a disproven belief that it was actually sealing something "in".