No, I have opened the fermenter before and it only takes a few seconds to build up pressure and start bubbling again. In this case I could see that there was pressure inside, just not enough to cause active bubbling. I have an S-shaped airlock and the liquid level was depressed on the carboy side right away.
Also, the RATE of bubbling was low. If it just needed to build up pressure, then after the first bubble the rate would have been the same as before. Instead there was an occasional bubble at first, with the rate gradually going back up to the previous value. That tells me that CO2 production was diminished temporarily.