On a cost level, yes it would make sense to leverage mstdn.ca but issue is that CBC wouldn't have the complete control & the instant brand recognition via hosting their own instance on their domain name.
It's also much better for the brand to have an account on social.cbc.ca. Something like cbcnews@social.cbc.ca looks better on a branding perspective than cbcnews@mstdn.ca. It's no different than how organizations use their own domain for emails.