The comments seem to be homing in on the issue -- it may not be discrimination per se, but rather, fiduciary responsibility. In the Q&A, both plans have the same trustee. For the one of the plans, the sole participant is also the trustee. Regardless of how poorly the investments that that trustee selects actually perform, I expect we can agree that a lawsuit against the trustee is unlikely. In contrast, the participants in the other plan are entitled to hold the trustee as the fiduciary responsible for whatever the investment performance turns out to be. After all, those participants were not offered the opportunity (per ERISA 404(c)) to select investments.
If the recommendation for "a platform for the employee plan" = using ERISA 404(c), I agree that is a sensible approach. Otherwise, the dentists should be prepared to defend the competency of the decisions made regarding investment of their employees' accounts. (Perhaps taking a pass on the bitcoin futures offering?)