Your surmise is sensible.
In the mid-1990s, many investment and service providers for governmental 457(b) plans were based in life insurance companies. (Most of the biggest players still are.) That’s why lobbying on what became § 457(g) sought to allow trust substitutes, do so with recognized forms, and include annuity contracts as trust substitutes.
When providers in 1996-1999 implemented the § 457(g) condition, many were done by adding one or two sentences of exclusive-benefit lingo to an annuity contract. When a governmental plan’s investments did not include collective trust units, some put non-annuity mutual fund shares under a custodial account.
A large governmental § 457(b) plan often negotiates for the recordkeeper to provide a trust company, which might be a subsidiary or affiliate, to serve as the plan’s directed trustee.