I'd also add that you may want to look closely at the fiduciary self-dealing PTs (in addition to the extension of credit).
If the DB plan is making a loan to Bob so that Bob can invest the borrowed proceeds into something Joe otherwise couldn't afford, I think you have a problem. For example, Joe wants to buy a piece of real estate for $500,000. He has $400,000. Bob agrees to borrow the last $100,000 from the DB plan to co-invest with Joe, allowing Joe to buy the property. Even if the loan is not directly prohibited under 4975(c)(1)(B), because Bob is not a disqualified person, it could still be fiduciary self-dealing on Joe's part under 4975(c)(1)(E).