That's right. This is true whether you are fully-insured or self-funded (assuming you purchase stop loss protection).
The fewer number of employees you have enrolled in your health plan, the bigger impact other employers will have on your health insurance renewal.
For an employer with 100 employees enrolled on the health plan, other employers might count 50% toward your renewal.
So, you might think your plan is running great but then you end up getting surprised by a double digit increase. That's because the other employers performed much worse than you.
Knowing this information is crucial when you start thinking in multiple years.