The wording "employee contributions" may be confusing in this case. However, employees may be allowed to make their own contributions into their SEP, effectively using the SEP as they would a Traditional IRA. I've confirmed this with multiple sources, including this morning in conversation with a retirement specialists at Schwab. Here's the precise wording from Vanguard:
Employee contribution limits
- Employees may be able to make traditional IRA contributions to the SEP-IRA of up to $5,500 for the 2018 tax year (or $6,500 if age 50 or older) and up to $6,000 ($7,000 for employees age 50 or older) for the 2019 tax year.
- This amount is the total contribution allowed by the IRS that employees can make to all their IRAs (SEP, traditional, or Roth) each year.
(https://investor.vanguard.com/small-business-retirement-plans/sep-ira)
