You used the word Roth all by itself.

Megabackdoor is when a 401(k) (employer plan) accepts after-tax contributions. These are only a contribution once. Then it is converted to Roth 401(k), but since it is basis, there is no taxable event. That's an In Plan Rollover.

Or, they took nondeductible 401(k) money and moved it to Roth IRA? That is a distribution and a conversion. Again, when it is only the nondeducted funds, it is Basis and that would be a nontaxable rollover. Hopefully, there was no withholding.

"The nondeductible contributions still must show up as excess deferrals"

Must? Or, does? Because there might also be excess deferral. That step is the first thing, when it is being contributed to the 401(k). Deferral is how the contribution step happened. Did you examine their allowed limit and all their contributions to the various parts of the employer's 401(k) plan?