I seems like fermentation should be finished by this point.
I think he meant that CO2 will come out of solution.
If diacetyl was produced in primary fermentation and a diacetyl rest was not done, the yeast will start to clean the beer if it is raised to 65F and you will see some activity. I've seen it with my own two eyes!
Germans don't do diacetyl rests. If the beer was fermented properly, a diacetyl rest is not always necessary. Also, I wouldn't think the yeast would become active just to clean up diacetly. That should only happen if fermentation is not complete.