I've done this race twice, both as a marathon and a relay.
As someone else said, it would be very easy to act like you finished a relay leg and drop out and drive off. You could then come back in and act like you were starting another leg.
The bibs differ in color and relay says the team name rather than your own name. However, with so many teams and runners going in and out, it would really be easy to sneak in and out. Only sharp volunteers might catch this.
Also, as a runner in a normal marathon of this size, usually you are aware of the others around you, but here with the relay, you don't know who is in what race. There's so much in and out that you'd never realize if someone suddenly jumped in and even if they did, you'd probably think they were just in the relay.