This rigidity in defining outbreaks and risk area is a bit silly in my opinion. Meteorology almost never has an objective set definition - a lone supercell producing 2-3 EF3+ tornadoes in my opinion is just as worthy of a moderate risk as 2-3 supercells producing one or two EF2-3 tornadoes, and in my eyes the moderate completely verified.
But I don’t think that having a scientific post analysis is a bad thing! In the case of this event, the environment was clearly favourable and able to produce violent tornadoes. The synoptic scale was supportive of intense discrete supercells, as one formed. I think there wasn’t really any particular reason for one storm as opposed to more, and just having one storm was the way this event coincidentally evolved. If we had 3 of these storms it’s not like we would be asking the question of “why weren’t there 4?”. At least that’s my line of thinking. I’m sure if an event like this were to happen almost exactly the same in some other hypothetical scenario, there would be evolutions with more than one dominant supercell. We are just lucky yesterday had one.