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True, but the GP suggested that you can find the absolute reference frame by looking at the relative velocities of the dots (independent of any force measurements).

The elevator analogy breaks down if you're larger than a point-particle in a point-elevator. Gravity's force varies with distance, causing tidal forces on your body, allowing something large/sensitive enough to feel the difference. Gravity stretches you whereas uniform acceleration does not, and non-uniform acceleration compresses you.

In other words, you can determine that you're in a gravitational field by measuring the difference in force at different locations in the elevator.

How would you tell whether that force is gravitational or centrifugal?

If it were gravitational there would have to be a mass in the right place to cause it. Maybe you could look at how the force changed as you moved around?