Yes, in fact that is how it works. If multiple independent contractors work for some firm, they each get their 30,000 exemption (if eligible) as individual sole proprietors of their individual businesses. They have their own business numbers and GST accounts which are considered individually.

Contractors are considered employees in some ways and not in others. This is mainly under labor ("labour", here in Canada) law which is there to protect workers from being exploited by being reclassified as contractors. Like you can't ask people to work in shit conditions just because they are contractors, and such, or deny them a lunch break.