But you have to look at the larger picture: The Federal prosectuion conviction rate is 95%. The Federal government is very good at getting prosectuions in cases they choose to prosecute. They tend to plea out cases that are riskier. A key tactic in doing this is they get one or more cooperating witnesses. That witness gets a better deal in exchange for their testimony that pretty much turns the case into a lock.
It's classic Prisoner's Dilemma. The first to take the deal is always the best off. Lawyers know it so if their clients are at risk, they'll tend to push their clients to be the one to take it. Ellison will most likely have direct knowlege of the situation and will be able to produce evidence such as emails and texts.
Importantly, she's not the main guy. It's SBF who opened up custodial assets to be used in that way (or so it seems). That's the primary offence here. It's a bit like how in a murder case the cooperating witness might be a driver or a witness but unlikely to be the actual shooter. They may get embroiled in a felony murder conviction otherwise thanks to the US-specific pecularity (and some, including myself, would call a miscarriage of justice) of felony murder but that's another story.
So Ellison fits the profile of an ideal cooperating witness, there's supporting circumstancial evidence and the fact that Federal prosecutors indicted SBF so quickly further suggests a cooperating witness with firsthand khowledge or they'd otherwise be mired in subpoenas for communications, conducting interviews and otherwise building a case.
A criminal indictment represents the end of the investigation, not the beginning. SDNY has already built their case to their satisfaction. People seem to think an arrest happens and the investigation continues. That's not how Federal prosecutions works.