It certainly isn't the amount of money they're investing
It actually partly is, super angels (as discussed in the essay) are investing more money than angels, but there is another distinction (mentioned in the essay) they are investing other people's money.
Somewhat paraphrased summary from the essay on the definition of super angels: super angels are VC's who invest less but act more like angels.
I think the definition is based on Super Angels investing other people's money (usually alongside their own money).
VCs - investing other people's money (endowments, etc)
Angels - investing their own money
Super Angels - investing their own money and other people's money
It actually partly is, super angels (as discussed in the essay) are investing more money than angels, but there is another distinction (mentioned in the essay) they are investing other people's money.
Somewhat paraphrased summary from the essay on the definition of super angels: super angels are VC's who invest less but act more like angels.