With the very important reminder that I’m not a medical professional, and this is not information I’ve committed to memory. Those numbers sound like the upper arm, rather than the forearm. Bleeding out occurs faster the more important the artery is. Meaning the further up the arm, the faster blood loss will kill you.
Conversely, the more remote the artery, the easier it will be to keep it effectively sealed, via pressure, and keep the victim alive long enough for help to get there.
If you’re talking about keeping pressure on the carotid artery (neck) or femoral (inner thigh) artery, you’re right, applying pressure to those is very much a loosing proposition. When you’re talking about the radial (forearm) things get a little strange. If you’re talking about a lateral cut, that is to say, the artery was opened in line with the arm, then those bleed out numbers are probably correct. You [i]can[/i] still control this with an improvised tourniquet, but it’s still very dangerous, and you’d probably need EMS there in order to properly apply pressure.
A perpendicular cut across the radial artery, severing it, is still life threatening, but it is comparatively easy to control. The pressure in the forearm is low enough that you can (effectively) stop the bleeding. There will still be leakage, it can still kill you (especially if the bleeding isn’t managed), but it is very survivable.
Also, completely severing an artery with a clean edge, will usually cause the artery to constrict, sealing itself against most bloodloss. It’s not a certainty, and if the artery is nicked or torn this can’t happen, but your body will do it’s best not to bleed to death, if at all possible.
Again, I’m not an expert. So, take all of this with a grain of salt.