For example, the linked answer might imply that yes you can just sprinkle salt on the baked brownie afterwards. But is that true? I also don't understand the comment about "raw flour". None of the flour should be raw after the brownie is baked!
Sprinkling salt on top won't have the same effect as mixing it in the batter, you'll get a big salt hit all at once and then nothing in the brownie itself which I wouldn't expect to be very pleasant.
Some recipes withhold some of the salt from the brownie and put it in a sauce for the top like a salted caramel drizzle. This works because it's not pure salt on the top and there's still salt in the batter.
If you forgot the salt, then you are kinda outa luck. However, a little kosher salt sprinkled on top and baked in can be quite Pleasant. Sweet and salt is a good combo.