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.
Americans, Japanese, Belgians, Indians and Eskimos also don't do diacetyl rests so long as they pitch the correct amount of yeast and ferment properly. However, with some strains a diacetyl rest is still recommended. See below from the Wyeast website:
"YEAST STRAIN: 2206 | Bavarian Lager
Used by many German breweries to produce rich, full-bodied, malty beers, this strain is a good choice for bocks and dopplebocks. A thorough diacetyl rest is recommended after fermentation is complete."
All I'm saying is if there is some, and the beer is raised to diacetyl temps, then there will be activity in the airlock. This just happended to me recently and the beer was feremented out. Could it have been degassing, I guess, but it lasted a few days.