He might've not explicitely said that, but from all said and shown in the movie, this is THE ONLY LOGICAL AND OBVIOUS CONCLUSION, safe from the craziest mental gymnastics.
Well either he guessed it, or more probably, saw where they were headed without FTL and it was the sole possible destination. Are there any alternative scenarios to that??!
That plot point has its own problems, and Boba Fett ain't one of them.
Either that, or it can't blow THROUGH the gas giant Yavin. Either way, they can't blow.
Having that said, that part has been made fun of ad nauseum, as well.
1) She spouts off expository dialogue along two other dudes, so obviously, she IS a leader of some sort.
2) Nothing separates her from that beard dude from Danduin, that cool other dude from Hoth, the beard guy from that same scene who looks like Obi Wan, and ITSATRAP. They're all, like... "leaders". Obviously. Maybe Mothma doesn't lead military operations, but just spouts off infodump?
3) It doesn't matter to the overall story, or anything else. It's the rebels. They are somehow organized, have like ranks, and some of their authoritative figures lead operations and hold briefings. Whether Mothma is "a leader", or "the leader", DOESN'T MATTER.