"If so, how was anyone saved before 1830."
People who are ignorant of the testimony of Joseph Smith have not rejected it. That includes everyone pre-1830 and the majority of people post 1830 as well. The statement doesn't apply to them, rejection requires knowledge.
Those who know it and make a choice to reject it will suffer the same fate as those who rejected Moses, or Noah, or any of the other prophets of God. Salvation comes from Christ, but those who reject His prophets reject Him.
You can likewise ask the question that since faith in Christ is required for salvation, what of those who lived before Christ. Are they not saved?
We believe that Christ did not limit salvation to only people who lived after his mortal ministry, salvation is available to all. If for one reason or another a person did not have a full and fair chance to learn and do all they were supposed to so that they could return to be with God, then they will get that chance between death and judgement. In the end, every person will have been given a fair and full chance, and any who do not make it will have only themselves to blame, not circumstance.
http://library.lds.org/nxt/gateway.dll/Magazines/Ensign/1977.htm/ensign%20january%201977.htm/the%20spirit%20world%20our%20next%20home.htm?fn=document-frame.htm&f=templates&2.0
Some people hostile to the church attempt to take the respect we have for the prophets and twist it into an accusation that we think Joseph Smith is somehow more important than Christ. It's hard to picture anything farther from the truth than that. The reason we have respect for Joseph or any prophet is because they are called of God, they are servants of Christ, not replacements.