This does sound like dangerous advice, unless they also tell you at which specific gravity to start bottling. If you bottle to early, you could get bottle bombs, and too late you get flat beer.
If you bottle at a SG close to the expected final gravity then you can reduce the chances of the above from happening. If you were going to use priming, sugar, for 2.5 volumes in 12 liters, you would use 3.2 * 2.5 * 12 = 96g sugar. (3.2 grams of table sugar gives 1 volume in 1 liter.) We can then extrapolate this back to a SG increase - table sugar is 36 gravity points per pound per gallon, or 8.314 * 36 = ca. 283 points per kilogram per liter. So the sugar would increase the gravity by 283*0.096/12 = 2.26 points. Since your extract beer kit is not 100% fermentable (unlike sugar) but probably close to 50-75% max, then you want to aim to bottle when the SG is at 2.26/0.75 = 3 SG above your final gravity. It depends upon the beer style - you could safely go up to 4 SG points above FG and end up with a well carbonated beer.
If the instructions don't tell you when to bottle or what the expected FG is, then I would simply let it ferment out and then bottle with priming sugar. It's safer and in many ways simpler than having to deal with all the guesswork about remaining fermentables in primary.
At 12 liters, the beer will be nearly twice as strong as the one at 20l, and possibly more bitter. You may want to simply get hold of some more bottles and brew the full 20 liters. Plastic 1.5 liter soda bottles work fine once cleaned (leave to soak for 24 hours in water then rinse and sanitize as usual.)
EDIT: In the comments, the OP realizes he's mis-read the instructions. The advice is then to follow the usual full ferment and add priming sugar - 96g in this case - as I recommended.