I'd say its an even split amongst respected fish keepers how many say that they can be alone and how many say that they can't.
There is a very simple answer to that issue. Observe nature and provide as close a copy as you can.
Natural habitat: Angelfish live in shoals. Within the shoal there will be a specific hierarchy. The fish are not stressed, because they have lots of room for the fish low on the pecking order to stay out of trouble. But the group remains in close proximity.
In the aquarium: A group of five has been scientifically shown to have less internal stress than fewer. A 4-foot tank is absolute minimum for this, with 5 or 6 feet being much better. In a 4-foot tank, fish may have to be separated in some cases. This largely depends upon the individual fish. While we can be assured of the "norm" for this or most other species of aquarium fish, there are always the exceptions.
So, maintaining a single angelfish in a 30g tank is not providing that fish with what nature intended, and for which the fish has evolved over thousands of years. I do not think it best to deliberately go against nature when that can be avoided. There is no doubt that the fish will be healthier in a more natural setting/environment.
Maintaining 2 angels together is only workable if they are a bonded pair such as when you want to raise fry. Keeping 3 is never advised, as one of the fish will always be the loser. Four fish carries this risk too, though depending upon the individual fish it can sometimes work.
Last comment, an observation of a true biotope display for this species, at the Vancouver Aquarium. A floor to ceiling (8 feet) tank, several feet deep (front to back) and wide (4 feet width at the front glass, but wider in the back). Very dimly lit, you have to stand in front of the glass for several moments to adjust your eyes to be able to see into the water. This tank contains a group of some 7 or 8 black ghost knifefish, and a shoal of about 10-12 angelfish that at present are about 4-5 inches in length. Pterophyllum scalare, the most common species and the one from which all variants have been derived. The angelfish shoal remains gathered around a vertical set of branches at one side of the tank close to the front. The knifefish swim among the rocks and branches at the rear and sides of the display. The angelfish have their hierarchy; watching them one can easily see the dominant male, but he simply looks toward the subordinate male and they turn away; the dominant ignores them further. The group remain within a few inches of each other, always. Rarely swimming other than to inspect the branches for food, and interact. This is how nature made this fish, and it is very instructive.