Explosion refers to the increase rate in those percentages, not to the percentages themselves. Example: expecting in a relatively short amount of time, say, 0.1% and getting 4% qualifies as explosion to me.
Sure, there's been an explosion in vaping, but no change to overall nicotine consumption.
> Obviously the rate increased, it's a fairly new device.
It's not necessarily so that "new device" therefore "rate increase." It could be flat or declining since earlier surveys. The fact that it isn't flat means that it's popular and likely warrants further study and/or intervention. The descriptor "explosive" applies here because many other similar public-health-impacting behaviors which are studied by similar researchers change much more slowly over time.