Your acquisitions lead is closer to the concern, and the reason has nothing to do with how sophisticated your client is.
Most state licensing statutes describe brokerage in terms of the activity plus the compensation. If you negotiate or attempt to negotiate the sale of real property for another person, for a fee, that's the description a lot of them use. A written service agreement documents what you did, and describing brokerage activity clearly in a contract doesn't move it outside the statute. Whether your specific arrangement falls inside your state's definition depends on that state's wording and on how its commission has interpreted it, which genuinely differs from state to state, so a real estate attorney licensed where you work is the person to answer it. That's a one hour conversation and it's cheaper than a cease and desist.
The reason the principal route exists is that when you sign the purchase contract in your own name, you're negotiating for yourself. You have a contractual interest in the property, and selling that interest is selling your own property right. Several states have added disclosure and, in some cases, registration or licensing requirements around assignments too, so the principal route has its own rules now.
One practical thing your lead probably hasn't raised. If you do get licensed, your compensation typically has to flow through your broker, and your brokerage may have its own policy on investor sourcing work and on how fees appear at closing. People get licensed and then discover their broker forbids exactly the business they got licensed to run.