> the fact that Twitter is a customer service channel

This is circular reasoning. Nowhere is it established that Twitter is a customer service channel.

Other parts of your in your first paragraph makes sense, but are also non-obvious and reflect expertise and evidence collecting on your part.

Plenty of large companies do also use Twitter for PR rather than customer service.

Unless clearly stated otherwise, there is no reason for anyone to treat an official Twitter account as anything other than an official statement by a corporation, no different from their official blog, or website.