Here is a decent discussion on the topic of AC versus DC IR:
So is this one:
From what I know, the manufacturers us AC IR for their testing. But I find that using DC IR, while not being directly comparable, has value in judging cell health over time, regardless of the raw values. As long as the device you are using has good repeatability of measurements. Something that many of the popular chargers are not real good at. The stand alone AC IR meters ae better at this.
Not really a problem, as long as one understands that, while the readings are correlated, they are not the same. But both can be used to help assess cell State Of Health.