Ven Buddhadasa's definition of "true Buddhists"

I read a book by Buddhadasa Bhikkhu titled “Buddha Dhamma for Inquiring Minds”. It is also known as Buddha Dhamma for Students. I’ve asked questions about this book before.

Buddhadasa says in the book that commoners are Buddhists in name only and that: “To be a ‘true Buddhist’ is to be ariya, those of noble practice who understand everything around them correctly to a far higher degree than ordinary people.” and: “The reward you will reap with nobility is to rise from the level of ordinary commoner to become a true Buddhist in the ariyan discipline.”

What do you think he means here by “true Buddhist”? My understanding was that anyone who takes refuge in the Buddha, Dhamma, and Sangha, and actually attempts to live by Buddhist principles, is a true Buddhist. By Buddhadasa Bhikkhu standards, wouldn’t most Buddhists not be true or real Buddhists?

I know the ariya sangha, such as stream winners, are more advanced and knowledgeable than commoners, but most Buddhists aren’t ariyas.

Thanks.