Further down they say it is 36,139 Arab American and allied voters who are signed up to their community pulse https://support.adc.org/a/communitypulse
I would think "our allies" would mostly be Muslim Americans. The two groups are separate but have some overlap and both vote similarly on Middle East and North Africa issues.
"ADC was founded by former U.S. Senator James Abourezk in 1980. Today, ADC is the largest Arab American grassroots organization in the U.S." Per their own about page, I think their numbers should be taken seriously.