That would mean that the 'Survey' would depend on the honesty of data submitters.
And data is collected only from the 'participating religious body'.
So, what % of participating religious bodies do you think participated and are you willing to accept the honesty of Muslims.
Also, how do you calculate adherents?
An adherent may never attend services.
I think this is just an obvious case of Muslims attempting to prove an unprovable clam.