When they say 3D printed do they mean a metal mill
It almost always means an additive instead of a subtractive process. Milling cuts things away and is subtractive. 3D printing sticks things together so is additive.
or can we 3D print with any random material now
Yes so long as you keep in mind that 3D printing describes a wide range of methods instead of a single one. In this case it's sintering metal powder one layer at a time with a laser. While expensive it's got some things going for it:
It's easier to produce titanium as powder instead of large solid blocks.
It's expensive to machine titanium from those solid blocks.
Titanium is expensive in general so methods like this with very little waste have an immediate benefit.
Laser are very cheap these days, give very good tolerances and don't really use all that much power. After these parts are made they probably don't need any more finishing than a bit of sandblasting.
And if so, why not use the far more tried tested, and better alternative milling?
Because we already know methods like this have several advantages over milling, electrochemical machining, casting, hot isostatic pressing, macro scale welding etc. It's not for everything but this application seems to tick all the boxes where laser sintering makes more sense than anything else.