Just tried this on my 289, with a 2.0Vp-p signal @ 1kHz, and it can't actually measure the duty cycle at all in DCV mode. It just says 'OL'.
With the meter set to ACV, it says 50.00% and 1.0000kHz, just as it should, and it works with the meter set to the 500mV range too.
I think the problem is with the manual. Frequency is an AC phenomenon, it doesn't make sense to measure a "dc frequency". Larger signals and lower frequencies do give a reading in DCV mode, but even so, it's not very accurate and I'm sure it's just trying to perform a measurement on a low-pass filtered signal.
I've also tried the exact same signal on my 89-IV, with the same result. It works fine on a 1kHz 2.0Vpp square wave if I put the meter in ACV mode, but not DCV mode.