Let me try another idea:

First 401(k) rollover to Trad IRA = if it was all pre-tax, it still is pre-taxed, and tax deferred.

Second 401(k) = if a new employer has a plan that accepts a rollover "into" it, the person rolled the Trad IRA to their Employer, and that would continue tax deferral.

Now we have No Trad IRA account. No SEP. No SIMPLE IRA.

Open a new Trad IRA account. Populate it with which funds, from where?

Backdoor Roth that Trad IRA account <== always taxable unless only Basis exists in the Trad IRA. Assuming the New Trad IRA account has only an amount recently deposited as Nondeductible, then that makes it all Basis (not time for earnings, even). Therefore, not a taxable event as long as there was no withholding, no earnings, and no amount held back by the owner, and it meets the timeliness rules for rollover/conversion.