It's not. Pressure cookers often max out at 10-12 psi, which will only get the contents up to around 235 - 240° F. Pressure canners are specifically designed to achieve 250° F at 15 psi, which is necessary to completely eradicate botulism causing spores.
True, the jars will definitely seal, locking in the contaminated wort.
Just because the lid "snaps" down doesn't mean the contents are sterile.
Certain vegetables can be safely canned in a water bath without being under pressure, because their acidic brine is inhospitable to botulism-causing spores. Sugary materials (like beer wort) are a completely different story, and require high temperature (250° F) and time to ensure all contaminants are killed. Since water boils at 212° F, the most practical way to achieve 250° F in a home kitchen is by using a pressure canner.
Boiling the wort kills almost all contaminants. ALMOST all.
The only way to ensure your wort is COMPLETELY sterile is by heating it to 250° F, typically in a pressure canner.
No, because the composition of the liquids are different. One is sweet and one is acidic. They have different pH values.
That's fine if you're going to freeze it. But if you're going to just store it at room temperature and treat it like a canned good, you are playing with fire, because it has NOT been sterilized. Since you're using canning jars with the "snap" lids, you will not get exploding jars (unless you screw on the locking rings), but it's only a matter of time before the wort becomes contaminated and unusable.
. I got to thinking about using the technique for an easy way to brew up a batch of light gravity wort & canning it for later use as a yeast starter...
; I only milled the grains once so the gravity came in at 1.025, so I needed time to boil it down to 1.040 before adding the cold water; & finally to boil off DMS? Don't know if it's necessary for a starter; but figured it wouldn't hurt anything. 
