You interpret this correctly. A 'one-man shop' is a business with only a single employee/owner, or only a single active principal, and in this case it clearly refers to the firm, not the firm's owner.
Technically, the supplemental 'verbless clause' Previously a one-man shop growing assets at a 5% annual rate is improperly anchored—since it introduces the sentence it should refer to the subject of the main clause, Edwards. But looking at this as a writer, I see that Ms. Swift is in something of a box: she wants to lead her sentence with a description of the firm as it was pre-transformation, but she also wants to focus on Edwards' action by casting her main clause in the active voice with Edwards as its subject. She consequently accepts the technical impropriety of a dangling clause—or perhaps she feels that in a one-man shop the distinction between the firm and its principal is trivial.