Yeah, not uncommon nowadays. Fidelity calls this option "BrokerageLink."
One way you will know it's not a separate account: you will still be charged your plan's management fees, even though you self-direct. These are usually rolled into the fund fees within the plan. For BrokerageLink, the irony is that these can't come from the BrokerageLink investments themselves, so they show up as a separate cash charge from within the plan. It does make these fees fully visible, though.