I know that is true for conventional powered welder. I know that either a generator or alternator can produce only AC, and the other can produce AC or DC. I thought that maybe it was an alternator, even though it is called a generator.
If the welder 220v powered it uses a transformer to increase the amperage. If the the welder uses a gasoline motor, the windings that make the electricity is an alternator. AC is easier to produce then DC, and it takes less HP to make it.
But since an alternator needs charge voltage to produce any output, a small DC generator is mounted on the end of the rotor to provide the charge voltage.
Here's a test you can try, take a GM alternator with a built in regulator, and spin it with an electric motor, and don't hook any battery to it, and check the output voltage, it will be zero, now add a battery to it and it will charge, the battery provides the charge voltage to energize the alternator.
Now try the same thing with a generator and regulator. The only difference is polarize the generator. Now with no battery hooked up to the generator, spin it and check the output voltage. It will charge without a battery.
The same principle is used here, the the DC generator starts to spin, it provides enough electricity to energize the stator in the alternator. An alternator will not charge a dead battery, but a generator will.
The biggest disadvantage to a generator, is they take a lot more power to spin, and have to spun faster to get the amperage out of them.