That would make sense, unless that 1.5m represent 4e players, and there are 6m D&D players. In which case, some 4.5m of the 24m lapsed players are actually current "old D&D"/retroclone/Pathfinder players, many of whom would be aware of the 4e changeover, but have decided against it for one reason or another. Another big chunk are probably "current gamers" who just don't play D&D any more (for whatever reason).
Remove those two groups, and that 24m people in the target market for the new Red Box suddenly looks a lot smaller. I do hope that this strategy works for them - the game could certainly use an influx of new players. I'm just not seeing this massive untapped market that WotC are banking on.
Wonder if WotC measured an itemized breakdown of that 24 million lapsed players into various groups, such as the two you mentioned and other ones such as older AD&D players who have not played any rpg games in over 10 or 20 years.
For example, out of the 20+ people I played D&D with back in the day, I'm only aware of 2 or 3 who still play any tabletop rpg games today. Many of these people stopped playing tabletop rpgs altogether after high school or college.
Certainly, the Red Box has no chance whatsoever of bringing me back to 4e - I tried it and decided against.
I imagine this would probably be the case for many, in that 24 million lapsed players figure.
The upcoming 4E "red box" being made in the style of the old Mentzer basic D&D box set, suggests WotC may possibly be targeting lapsed gamers who played D&D back in the 1980's but whom today do not play any tabletop rpgs at all. These may be people who have not played any tabletop rpgs at all over the last 20+ years.
I don't think that 4E "red box" is targeting people who are current or recent players of 4E and/or 3.5E/Pathfinder. Targeting people who currently or recently played the retroclones and older editions of D&D/AD&D, is probably not their main aim either.
If they are indeed targeting people who have not played any tabletop rpgs in over 20+ years, they are most likely looking at a demographic of people over age 30 to age 50+. A 40-something former gamer today, would have been in high school or college when the Mentzer red box was first released.
It will be interesting to see how WotC will advertise this 4E "red box". I suppose they could put up television ads on a cable channel like Spike and/or during prime time shows like CSI, if they are targeting the over age 30 to age 50+ crowd.