As I read it, nothing has changed, except that it more clearly defines that the T/C Encore kits are now legal, and further defines HOW they may be assembled.
To extend the letter of the rulings to apply to other firearms, here is an example, following the letter of this brief:
I can buy a Ruger Charger Pistol, then add a 10/22 rifle stock, and a 16"+ rifle barrel, and legally convert between the two. The 4th page specifically states that refitting a "handgun" into a rifle (16"+ barrel AND shoulder stock over 26" total length) does NOT constitute "making" a firearm, which would be regulated. It also holds that converting that new rifle BACK into a Charger would not constitute "making" a firearm, because the weapon STARTED as a handgun.
HOWEVER, if I start with a 10/22 rifle, I CANNOT add a Charger 10" barrel and a pistol stock, without re-listing the weapon with the NFA office (again, on page 4). This is how the law always has been. You can "up-convert" a handgun into a rifle, however, you cannot down-convert a rifle into a handgun without re-listing the weapon.
ALSO, on page 3, it confirms that even legal kits cannot be possessed in an SBR form, i.e. rifle barrel on a pistol stock, or 10" barrel on a shoulder stock. It confirms that these kits can be assembled as a RIFLE or a PISTOL, but NOT as an SBR. At the bottom of page 3 and top of 4, it specifically states that SBR's, or weapon parts in close proximity that could be assembled in that fashion, are regulated under the NFA. Basically, if you had ONLY 10" barrels for an AR-15, and ONLY rifle stocks in your possession, even with AR-15 "pistol" listed receivers, these would constitute SBR's, and be in violation of the NFA. HOWEVER, if you had pistol grips, rifle stocks, 10" barrels, and 16"+ barrels, AND pistol listed receivers, you are in the clear, so long as none are illegally assembled into SBR's.
So again, these rulings basically more clearly define that 1) rifle/handgun kits can be converted back and forth without recourse, 2) a handgun CAN be converted into a rifle (and back again) without re-listing, but 3) confirms that a rifle CANNOT be converted into a handgun without re-listing.
Specifically to answer your question, you cannot add a shoulder stock to your glock WITHOUT A 16"+ barrel (which doesn't work so well with a short-recoil weapon). Again, refer to the page break between pages 3 and 4. Short barreled rifles STILL fall under NFA registration and classification.
On a side note, in my own shop, on jobs where I have converted Chargers or AR-15 pistols into rifles, I am ALWAYS very careful to remove the short barrels FIRST before assembling the shoulder stocks (and removing long barrels before fixing pistol grips/stocks). The last thing I want is to get surprised by an inspection and have an "illegal" SBR that I hadn't finished converting INTO A LEGAL RIFLE sitting in my shop.