The reasoning behind this is that, when you're carrying inside-the-waistband, barrel length is largely irrelevant to concealment, while grip length is very relevant. So theoretically the "19L" has all the velocity and sight radius and other benefits of the full-size gun, with the shorter more concealable grip of the 19. This copies the old "Concealed Carry Officer's" pistols from Gunsite, which married a Commander-length slide to an Officer's model frame for the same reasons.
Now Glock is selling what is basically a commercialized version of their MHS entry as the "Glock 19X", with the 19's compact slide atop the 17's full-length grip. For the MHS entry, this was because the contract specified both a 17-round magazine capacity as well as a maximum overall length.
A lot of the shooters I know are having a field day with this, with parodies like this one popping up all over social media:
The general shooting public vastly outnumbers the tiny demographic slice who even know what a "19L" is.
The general shooting public likes 1911 Commanders, 2.5" K-frame snubs, and putting Pearce grip adaptors on the G22 mag in their OWB-carried Glock 27. They buy truck holsters and Remoras.
Given how many people carry their SR40 and XD40 subcompacts with the (included) full-size magazine and (included) adaptor sleeve, Glock shows it knows more about the general shooting public than any of us 19L-loving snobs. They'll buy this.
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