Yes, part of the process during the investigation would be to substantiate claims made and as a part of that process they had to make a FISA application. If they already knew everything then they would not need to do further investigation.
Fifteen pages into the request they bring up Source #1 and say (and I am paraphrasing here) Source #1 has given us reliable information in the past and based on previous history with this source we believe his reporting to be credible. Although Source #1 is subequently mentioned several more times This is just one page of the 31 pages of discussion regarding Page and his Russian associates that are outlined as a reason for further investigation.
Page 32 begins the conclusion wherein it states that based on the forgoing facts (31 pages worth) they believe that Page has been “collaborating and conspiring” with the Russian government.
Also, the document is heavily, heavily redacted and without knowing what is under those heavy black lines, what the FBI knew or didn’t know is mere conjecture on our part.
And, unfortunately neither you nor I are privy to just how much of the original dossier has been verified so to try to describe it as “unsubstantiated” is somewhat disingenuous.