"Everyone" in this case means anyone who matters. People who finance other trips and set off a series of events that make use of the "discovery".
The key words in my frist statement are "for that society". The American Indians had no contact with Europe. The Europeans had no idea that this continent was here. Yes, Leif came to Greenland and there were two settlements that lasted about 100 years, but when the villages failed and the people there died, it was hundreds of years and many generations before Columbus. By that time the European society knew nothing of the previous discovery. So. If an entire society knows nothing about a certain item or place, and some one from that society finds all about it and tells that society, and by that little snippent of information, changes the society, didn't that person "discover" the place or thing for that society?
Let's take another tact. The Mayan, aztec and the Inca cultures never "discovered" the use of the wheel. They were isolated from Europe and Asia where the wheel had been used for millenia before. If some one in the aztec civilization had "discovered" the use of the wheel would that have been his discovery or not?
This is not the same thing as saying X was the first to climb Mt. Y. That is a finite thing.
Essentially the person who gets credit for discovery is not the first person to see it, it's the first person to see it and DO something about it in a way that changes the entire knowledge of a society.
Leif discovered America for the Vikings and a few settled in Greenland and maybe what is now Nova Scocia, but within a generation, the village was gone, the society was unchanged and then again no one in power of who cared new anything about it. It was only after the state-sponsored voyage Columbus that rapid changes hit the European society.
See ya
Tony