The prevalence of severe diseases caused by recessive defects in ADA — severe combined immunodeficiency and ADA deficiency — is extremely rare (1 / 100,000 is a high end estimate). This variant was seen in 9 out of 128 random PGP + public alleles, contradict such an effect.
To demonstrate significance, we compare observations against the allele frequency that would be expected for a pathogenic variant causing ADA deficiency. A variant responsible for 20% of disease would be expected to have an allele frequency of .06% (.2 * (1 / 100,000)**0.5). The chances of seeing such a variant 9 times in 128 random alleles is extremely low (4 * 10^-16).