Well - it was the wild west back in the '90s for §403(b) arrangements. Looks like that section applied for plan years 1988 and later, and clarification came out in 1989 - so I'm guessing the document provider didn't restrict it.
This paper is from 1995 - but explains the earlier history pretty well I think.
https://www.irs.gov/pub/irs-tege/eotopici95.pdf
"Prior to 1986, there were no nondiscrimination rules applicable to 403(b) plans. TRA '86 added separate nondiscrimination rules for non-salary reduction and salary reduction contributions under clauses (i) and (ii), respectively, of IRC 403(b)(12)(A). These rules generally must be satisfied in operation for plan years beginning after December 31, 1988. Pending the issuance of regulations or other guidance, Notice 89-23, 1989-1 C.B. 654 (extended by Notice 92-36, 1992-2 C.B. 364), provides guidance for complying with the nondiscrimination rules. Specifically, Notice 89-23 deems a 403(b) plan to satisfy nondiscrimination if either the employer operates the plan in accordance with a good faith, reasonable interpretation of section 403(b)(12) of the Code, or in accordance with the safe harbors set forth in the Notice. A. Salary Reduction Contributions Salary reduction contributions are tested separately for nondiscrimination under clause (ii) of IRC 403(b)(12)(A). Nondiscrimination with respect to salary reduction contributions generally is satisfied only if each employee may elect to defer more than $200 annually. Thus, for salary reduction contributions to a 403(b) plan, there is no nondiscrimination analysis of the amounts contributed. The test focuses on eligibility and generally requires universal eligibility. There is no requirement to offer the opportunity to make salary reduction contributions. Once that opportunity is offered to any employee, it must be offered to all employees in order to satisfy this requirement."