True, but that is beside the point. Cyclic rates on full-auto-capable firearms don't take into account mag changes either.
Based on what I've seen in person (30-round dump in 3 seconds), and on YouTube (30-round and 100-round dumps in 3 or 10 seconds, respectively), if you had a 150-round mag, and a bump-stock, it is entirely possible to dump 150 rounds in 15 seconds.
Well, you HAVE to take into account mag changes for the purposes of evaluating this particular issue. This guy Davis said that an AR-15 could fire 150 rounds in 15 seconds. Well, let’s see the YouTube of someone actually doing that. The M16’s cyclic rate of fire of approx. 900 rounds/min. is unrealistic for a full minute...because even if you had a 900-round mag (or, more likely, a belt with 900 rounds in it), the barrel would melt and/or the action would seize well before you got to 900 rounds. Cyclic rates of fire are nice in theory, but meaningless except for very short periods of time.