So many people are told they can't sing - by music teachers, friends, family. I'm with the premise here that everyone can sing: pitching the voice is a learned skill, as is holding a rhythm. The quality of the voice is to some degree a physiological thing but can only improve with guidance and practice.
Formal music often calls for a voice type that is constrained into an unnatural way of singing (still sounds great IMO) but lullabies, work songs, pub songs, playgroung chants etc can be joyous and very musical
I sang in the 'remnants' at school, and was devastated when I realised that it wasn't a compliment.

Stu